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High Build Primer

AviatorJ

Well Known Member
I started to post this in the 'Primer' section but then it occurred to me it has more to do with Fiberglass work than the paint itself.

Today was my first time to use high build primer. I've been using the cabin top interior as a bit of a testing ground to learn various techniques working with Fiberglass. While it looked somewhat smooth after working with various fiberglass micro, flox and skim coats... after spraying with high build primer the imperfections really stand out.

My understanding is after these first initial coats I basically sand these areas smooth and then reapply the high build primer. There was also mention of block sanding but all these areas have curves or are concave, which doesn't seem like blocking would work on. Was going to try and hand sand these smooth with some 50 grit, then work some new micro in and then try the high build primer again. Does this sound doable or is there some other way I should approach this.

Here are some pictures.





Thanks,
 
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You are right, judging from the pictures, you are still in the fiberglass filling and sanding stage and way too early to apply high build primer.
The inside of the cabin top on the RV-10 is a particularly unwieldy place to fill and sand. As you noted, most of the areas are concave and rough at that.
Best to apply several light coats of filler to smooth out the roughness and sand in between. The inside of the cabin top really takes an enormous amount of
elbow grease to get the finish you want.
This is also a good time to decide if you will have some kind of a headliner or overhead console. Adding a headliner will save you some of the sanding chore, at least in places that would be covered by a headliner.
The good news is that the outside of the cabin top is convex and nicely finished,
requiring much less filling.
 
Not ready for primer yet

You are right, judging from the pictures, you are still in the fiberglass filling and sanding stage and way too early to apply high build primer.
I agree. I would fill in the big holes with Polyester Glazing Compound [Evercoat] and then sand with small pads starting with 80 grit and working your way down. It builds character :rolleyes:
 
As always thanks so much for the replies.

I do plan on using the Aerosport headliner and I already installed their center console. I will be painting the front pillars and the upper sides by the doors, so that's what I'm going to focus on getting pretty.

On the previous fiberglass pieces I used skim coats (Just resin) to fill in pin holes with really good success. When you are talking about 'filler' are you referring to micro and resin combo? Or is it a 'bondo' like product or this polyester Evercoat stuff?
 
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I know I am going to get flamed for this but I used a combination of fillers depending on the stage of the project.
For the rough filling and high build up areas I used the lightweight epoxy micro filler, or whatever has become your preferred solution at this point.
For subsequent filling of shallow spots I used lightweight polyester filler "automotive type Everglaze, Bondo " for the sole purpose of expedience.
This type of filler is heavier than micro but it is sandable in about 30 minutes.
Therein lies the expedience where you can start on Saturday morning and pretty much finish the project by the end of the day, rather than apply epoxy type filler and wait for the next day which may not come for another week for those of us who have other obligations.
Following up with filling and sanding perhaps as many as 3 or 4 coats in a day puts you in the "zone" and allows you to pursue that nasty fiberglass job with a vengeance.
In the end, you will have added an extra few oz over using the lightweight choice of epoxy micro filler.
 
No flame here!

I know I am going to get flamed for this ...
Why? What you described was the correct way to do the job. Sure, the Evercoat is "heavy", but you only use a minuscule amount of it and you can sand it in 30 minutes. Finishing fiberglass is a nasty job and you want to get through it as fast as possible. Ask me how I know :(
 
When you think you're ready for primer, spray a very thin mist-coat of rattle-can black sandable primer and sand it all off. It will show all the defects, pits, and pinholes.
 
Why? What you described was the correct way to do the job. Sure, the Evercoat is "heavy", but you only use a minuscule amount of it and you can sand it in 30 minutes. Finishing fiberglass is a nasty job and you want to get through it as fast as possible. Ask me how I know :(

"Correct" would be a stretch. Nothing about polyester performs as well, long term, as epoxy/micro.
 
I'm having flashbacks!

Nothing about polyester performs as well, long term, as epoxy/micro.
I've used both and had no problems with either in twenty years. I guess I should have qualified my comment that you only want to use the polyester in thin layers or small diameter holes (like pinholes). If the hole is deeper or wider than...say...1/32", then micro would be advised. It's been my experience that micro is harder and more difficult to feather into the surrounding fiberglass than Evercoat, but YMMV. I've also used West Systems + microballoons over wide areas on my wings (It's a Lancair, remember?). The West Systems micro is softer and easier to sand than standard micro, but you want to make sure to put it on thick enough so that you don't need a second application over a low spot. What happens there is that the second application leaves a ring of pure West Systems around the edges that you can never sand out without damaging the surrounding areas. Also, I've had small areas of the West Systems + micro collapse over the years - due to entrained air bubbles, I assume. I had to fill those areas with Evercoat then spray with high-build primer (I used WLS Systems). I've never had any of the three products delaminate. Surface preparation is critical there, I think. This conversation is giving me horrible flashbacks :eek:
 
I'll stick with micro and resin to keep it simple. Plus it's what I have on hand right now. I don't mind the wait, keeps me from feeling rushed on things.

This morning I went to town and spent a good hour or two sanding away on things. Then I mixed up a batch and played sculpture artist for another hour. I did go to an auto parts store and bought some bondo spreaders and some sanding blocks. The spreaders worked better than what I was previously using, won't be able to use the blocks till this new micro application hardens.

The pillars and non flat areas are a bit of a mess. I'm guessing this is where the sanding and reapplication of micro comes into affect... and then I just keep repeating the process until I'm happy with it all. Guess we'll see where I'm at on it in a week.
 
Follow up questions - I've done two or three Micro filler coats and in between those used 60 grit to help shape it. In some area's where I didn't want to sand off too much material I used 150 grit.

After this mornings daily sanding session I now fell the shape is generally what I want. So I mixed up some resin with a little bit of cabosil and brushed it on all over the place. Spent about 15 mins just walking around and re-brushing everything as the resin began to thicken up a bit.

What are some suggestions past this point? Here's what I'm thinking...

- After this use 180 grit and give another sanding, then another resin coat.
- Then sand 320 and apply the high build primer.
- Once cured look for any imperfections or high/low points and work on those specific areas with more micro or resin
- Once happy reapply primer as needed
- Sand 320 and top coat with paint

Thanks for the additional help.
 
Here are some updated pictures. I'm not completely happy and will work on the detail of some of the areas but it's much better than it was. Thanks for all the help.



 
I would recommend you watch the videos by Mike Arnold on how he built his AR-5. He used to sell VHS cassettes but before he passed away a few years back he posted them all on youtube. Some good information on sanding and filling. He built flexible sanding blocks out of hard rubber and plastic. He used epoxy micro on flight worthy parts and bondo on patterns and plugs. But he also used high built catalyzed polyester primer over the epoxy micro so he did mix both resin types. Paint was usually imron.

When I am sanding compound curved surfaces, I used circular sanding motion to ensure that I don't leave any flat spots. If you are unsure of whether your surface is perfect, use your hands to feel it - they are more sensitive than the eye, depending on the lighting. And/or, take a light and shine it at a very low angle to the surface - that will make any imperfections cast shadows. I did a lot of drywall work at home and I would finish something late in the evening and it would look great. Then in the morning natural light, which is low, all the flaws would show up.

If there are flaws, paint will accentuate them - it won't hide them. And if you can feel it, you will see it. So you just keep sanding and filling until you just can't stand it, then do it a bit more. Only real wierdos (like me) enjoy that part of the build :)
 
Another good way is to spritz some black paint on the surface( cheapest you can find). Then sand away in circular motions until no more black. Works good to see all the highs and lows. It's called a guide coat for blocking.
 
Follow up questions - I've done two or three Micro filler coats and in between those used 60 grit to help shape it. In some area's where I didn't want to sand off too much material I used 150 grit.

In the future, try to apply enough micro the first time to do it once. It doesn't always work out that way, but it is a worthwhile technique. Second, third, and subsequent applications are unlikely to duplicate the resin-to-microbubble proportion of the first, so they will sand differently. As a result, it will be harder to sand a surface perfectly flat.

After this mornings daily sanding session I now fell the shape is generally what I want.

Novices tend to approach surface finishing as if some later coat of something will fix imperfections which are clearly apparent right now. The result is layers and layers of mixed stuff in an endless cycle of sanding despair.

Adding more layers is not the answer.

Instead, (1) build a highly accurate mold or form, (2) used to lay up accurate glass which needs little filler. (2) Fill whatever minor lows remain one time and perfect the shape, (3) seal, and (4) shoot primer.

What are some suggestions past this point?

Start over.

Sand off the resin/cabosil layer. Finish all the contour work in micro. Then, and only then, skim coat with neat epoxy. Sand gently when cured, just enough to slick it out. Do not sand through into the underlying fiberglass or micro. Then shoot a urethane high-build.
 
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Another good way is to spritz some black paint on the surface( cheapest you can find). Then sand away in circular motions until no more black. Works good to see all the highs and lows. It's called a guide coat for blocking.

Save yourself a lot of trouble and just use a pencil.

Blocking%20Scribbles.JPG


BTW, there's a good use for Evercoat Rage Gold. Sticks to plywood really well.
 
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In the future, try to apply enough micro the first time to do it once. It doesn't always work out that way, but it is a worthwhile technique. Second, third, and subsequent applications are unlikely to duplicate the resin-to-microbubble proportion of the first, so they will sand differently. As a result, it will be harder to sand a surface perfectly flat.



Novices tend to approach surface finishing as if some later coat of something will fix imperfections which are clearly apparent right now. The result is layers and layers of mixed stuff in an endless cycle of sanding despair.

Adding more layers is not the answer.

Instead, (1) build a highly accurate mold or form, (2) used to lay up accurate glass which needs little filler. (2) Fill whatever minor lows remain one time and perfect the shape, (3) seal, and (4) shoot primer.



Start over.

Sand off the resin/cabosil layer. Finish all the contour work in micro. Then, and only then, skim coat with neat epoxy. Sand gently when cured, just enough to slick it out. Do not sand through into the underlying fiberglass or micro. Then shoot a urethane high-build.

Fantastic advice which I literally learned this week the hard way!

Was out at a good family friends garage who restores and collects high end antique cars. His latest project is an early 1900s Packard I believe. The frame and molding are completely hand formed wood and when I saw it a few months ago it was in rough shape and now it's completely smooth and they're in the process of painting it. Amazing what they are going through to get this think near perfect. That started an entire conversation about painting and filler and we talked about my plane project. He showed me some of the tools they use and the big take away is shape everything to how you want it will a low grit... then use subsequent higher grit sand papers to smooth things out. For some reason I had in my head that higher grits are to further refine the shape you're going for. As you pointed out this got me stuck in this cycle of constant sanding and adding and sanding etc.

I was also mis judging how much filler to use. I'm working on the wing tips right now as well and had ended up sanding off way too much so the lenses were no longer flush. So I gobbled what I felt like was a lot, but still had to use two applications to get it all there. I should work on molds, not sure what I would use for a reverse type mold.. maybe cut foam that I can then push down onto the top.... I tried using peel ply a few times to push filler into a shape, worked somewhat descent, still had a ton of sanding to do.

On starting over sanding off the resin coats, I actually did that yesterday on the wing tips. I just wasn't happy so used some 80 to remove the skim type coat and then stepped up to 150 and then 320. Shot with some high build primer in some areas to see what I could smooth out and then this morning used 80 to clean it all up... then stepped up again and shot with a few coats of normal primer. Tomorrow or I guess later today I'll look it over again, fill or sand any little areas that need to be fixed and reprime though areas. Then those puppies are done!

You know I was dreading the fiberglass work but now I'm actually enjoying it. Has a bit of an artistic flare to it and I feel like I'm improving. I imagine I'm wasting a lot of resin, paint and paper but it's probably cheaper than a class.
 
As you pointed out this got me stuck in this cycle of constant sanding and adding and sanding etc.

Apparently you're still stuck:

I just wasn't happy so used some 80 to remove the skim type coat and then stepped up to 150 and then 320. Shot with some high build primer in some areas to see what I could smooth out and then this morning used 80 to clean it all up... then stepped up again and shot with a few coats of normal primer. Tomorrow or I guess later today I'll look it over again, fill or sand any little areas that need to be fixed and reprime though areas.

There was no need for 320 grit until much later in the game. All contouring (working with filler) can be done with 80 and 120.

High build comes much later (see below). The only thing that goes on over high build is finish paint; the color and (if desired) clear coats. What you're doing above is the "layers and layers of mixed stuff" previously described, which is bad procedure on several levels.

There is no reason to spray anything until the part is entirely filled, contoured, and sealed....a done part. Up to that point, use nothing but a quality filler. Opinions vary, but I'd stick with epoxy and glass micro over an epoxy matrix composite. Polyester/micro/talc/styrene premix auto body filler is fine over polyester matrix composites or gel coat, or just use epoxy/micro there too.

The internet is full of advice about spraying guide coats so you can see the low spots. Maybe it's necessary if you're working by firelight in a cave. For the rest of us, a few pencil scribbles does the same thing, and doesn't contaminate the surface with mixed chemistry, or clog the sandpaper. You don't even need the pencil if the part can be viewed with flat lighting.

See the shiny spot in the middle of this panel? It's a low spot where the sanding block has not cut. It needs filled, or the surrounding material needs cut down to the same level.

Contouring%20Micro.JPG


Pencil%20Marking.JPG


After the part is entirely on contour, you'll need to kill the pinholes. I like an epoxy skim coat, sanded slick without breaking through. Some guys are using drywall mud, squeegeed into the surface and sanded until no trace of it remains except down in the pinholes. Azko sells a special pinhole fill product, as does Lohle. The drywall mud, Azko, and Lohle methods are all pretty much the same concept; after application, the product remaining down in the pinholes is locked into place by a subsequent application of primer. However you do it, fill pinholes before spraying any kind of primer.

High build primers are used as a final step, just before color. Their purpose is very fine adjustment of surface contour so it is optically flat, i.e. no waviness is apparent in the gloss-painted surface when viewed at a low angle. High build is not applied until after the part is entirely contoured and sealed. If you measured the surface contour of the bare part, the highs and lows would be perhaps 0.005" difference, maximum. That's the build thickness in "high build"...a few thousandths. For example, PPG K36 is sprayed in 4 mil coats (0.004"). After cure and blocking, the desired dry film coat is 1.5 to 2 mils (0.0015 - 0.002"). If you have low spots measuring 0.003 deep, it will take two or three rounds of high build and block sanding to get it perfect.

How to use it? With the surface entirely contoured and sealed, the best approach is spraying a dark color epoxy primer followed immediately by a lighter color urethane high-build. The wet-on-wet approach ensures that the urethane high build is "glued" to the sanded epoxy surface, and the color contrast is a sanding guide.

The cured high build is block sanded with hard blocks, using 400 grit wet-or-dry paper. The idea is to skim off the high places and leave the lows untouched, thus the hard block. Sand evenly across the entire surface until the dark epoxy primer just starts to peek through at the high places. Stop...do not cut into the dark primer! Spray another 4 mil cross coat, cure, and block again. The process repeats until blocking the entire surface does not result in dark epoxy primer showing through anywhere, while the former lows are now built up enough to be cut by the sanding block.

Glass canopy frame, at the high build stage. See the dark primer peeking through at the highs? Time to spray another cross coat of high build.

Canopy%20Blocking.JPG


Canopy%20Int%20Finishing.JPG


I should work on molds, not sure what I would use for a reverse type mold.. maybe cut foam that I can then push down onto the top.... I tried using peel ply a few times to push filler into a shape, worked somewhat descent, still had a ton of sanding to do.

Female molds and male forms have nothing to do with finishing factory supplied parts.
 
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Recommendations for sanding a tight inside radius?

Dan,

Any recommendations for sanding a tight inside radius? The part I'm working on is the transition inside the top of the RV-10 canopy above the rear door pillars.

This part comes from the factory very ugly. I'd say the radius is somewhere between 1/2" and 3/4"...

Thanks!
 
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Apparently you're still stuck:

Indeed I am... my wingtips are good but when I put primer on my cabin top this morning it looked worse than I thought. I haven't developed an eye for this yet. Tomorrow I'm going to have a friend over and we're going to go crazy with low grit sand paper to try and get the shape there. I'll try the pencil trick and do it outside in the sun and see how well I do at it.

As always thanks for your input, your work is beautiful. Am curious about the small spots as well. As Eric mentioned some of these are tight areas. I had tried to hand sand in little circles which looked fine... until the paint.
 
Dan,

Any recommendations for sanding a tight inside radius?

Patience, mostly. I have a cardboard box full of sanding shapes...short lengths of tubing in various diameters, little blocks of wood, foam blocks, etc.
 
Sand off the resin/cabosil layer. Finish all the contour work in micro. Then, and only then, skim coat with neat epoxy.

It took me about 6 hours to sand off my various layers of micro, resin, paint, sweat and shame. I took everything down until I once again started seeing pink. Built up with Micro filler any areas that needed it and then using nothing higher than 80 I contoured everything using various shapes and sizes. I still had to do some by hand because I don't have shapes that would work, it the inside of the cabin pillar.

This afternoon I'm going to go over it again with fresh eyes and if I don't see any issues then start the skim coat. So I have a question about that... previously I did skim coats with straight up resin and a flat piece like a bondo spreader. I would put it on, wait a few minutes and then 'skim' any excess off.

How is this done on non flat shapes like the inside of the cabin top? Would it work to take a small brush and more or less paint it on... give it a few minutes and move the brush around trying to collect any excess? I could do this until it starts to get tacky and hopefully prevent any types of runs.

Thanks!
 
Squeegee it into the surface, then roll it out. Try a 3" cheap nappy roller, or a foam roll. Leaves an orange peel surface finish, which when cured is a great sanding guide. Sand with ~180 paper just until the pebbly look is gone, and no more. Do not break through into the underlying glass or micro.

Orange%20Peel%20II.jpg


This afternoon I'm going to go over it again with fresh eyes and if I don't see any issues then start the skim coat. So I have a question about that... previously I did skim coats with straight up resin and a flat piece like a bondo spreader. I would put it on, wait a few minutes and then 'skim' any excess off.

How is this done on non flat shapes like the inside of the cabin top? Would it work to take a small brush and more or less paint it on... give it a few minutes and move the brush around trying to collect any excess? I could do this until it starts to get tacky and hopefully prevent any types of runs.
 
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Thanks once again sir!

I have some 4 inch foam rollers so I used one of those. Tried to squeegee it on but wasn't able to get the inside curves of the door pillars. Ended up using a brush to put some resin in place. Then I went over it a few times with the roller, which was soft enough to work the contours. Will have to wait till tomorrow to see how it all looks.

Edit: I found several low spots after the skim coat.. HB primer may have filled it in, or may not have. Heeding Dan's advice I'm going to sand off the skim coat and refill with some clean micro.. will sand and reskim.
 
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Eric asked about sanding in tight places. Here's a tight place. It's a duct for an internal oil cooler in a Yak 50, something to finish up before it gets cold.

With all the aluminum structure in place, the duct walls were fabricated from 1/2 pink foam panels carefully cut to shape. Each then got 4 plies of 9oz plain weave on each side. About 1/4" of foam core was removed along the edges, then they were floxed into place, filling those edges with flox core bonded to the aluminum skin. A 1/2' flox filet was wiped into all the internal corners, with solid flox fill at the leading edges of the side panels.

After cure, cleanup sanding, and some leading edge shaping, the interior surfaces got a single ply of 9oz extending outside the inlet and lapping on the the outer skin. After cure and trim, it was time for finishing filling. Bring on the micro:

Micro%20800w.jpg


Sanding tools are, as previously noted, whatever works. I make most of my blocks from wood and use 3M spray contact adhesive to attached good paper, here 80 grit 3M Green. I've also been known to grab a gold stickit disk (normally used on the air sander) and wrap it around whatever shape I need; note length of fuel hose with the right radius to match the internal corners. A length of aluminum tube stuck inside the rubber keeps it straight, and provides a handle.

Tools%20800w.jpg


Most of it is done with hard flat blocks, as the idea is to make the surfaces as flat as possible. The only filler is micro, and we're still a long way from any kind of primer.

Sanded%20Duct%20800w.jpg


When all surfaces are satisfactory (no flaws...here fingertips will find them better than the eye), they get sealed with a straight epoxy, here low very viscosity Clear Coat from System Three, applied with a foam roller.

Skimmed%20800w.jpg


When cured, this will be scuffed with 220 or higher, without cutting through, until dull all over. Then, and only then, it's time for primer. I'll shoot a cross coat of PPG DPLF immediately followed by a high-build, if I decide to take finish to a high level. Probably not; it's a Yak, and they tend to be more like a well used axe than a fancy straight razor ;)

BTW, stopped by Lowes on the way to the airport yesterday to pick up a cheap foam roller. They didn't have the really cheap kind I was looking for, but I did notice a new product, a foam roller about 1" diameter with a single sided axle. The other end is a clean hemisphere, nice for rolling into filleted corners. So I blew $3 on the experiment. Darn thing worked great.

https://www.lowes.com/pd/WHIZZ-Mini-Paint-Applicator-Kit/3013611
 
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In the Mike Arnold videos he covers the entire airplane, or part, with micro and sands with 36 grit to knock off the lumps and achieve contour. As Dan advises he takes it down to 120, filling in any major blemishes, then applies high build primer.

For sanding high build primer I block the first coat until I see that the sanded spots make up about half the overall area. Then with the next coat I increase the percentage, so that coat is filling the valleys. You want every part of the surface to have contacted the sand paper but you don't want to cut through the primer anywhere so it is a balancing act. The best way to achieve that is to use the primer only to fill scratches, not to compensate for poor contour. If you have good contour you won't have to apply many coats of primer to hide your sins.

I think what I am saying is in line with Dan's comments. I agree with everything he has written.

Incidentally, Mike Arnold worked in the sailplane business for years and on his AR5 he achieved levels of surfave waviness that were extremely low when measured, allowing him to have a large percentage of laminar flow. He really had this stuff down to a science. I knew some of this stuff but have not typically piled on the micro like him. But I also never used 36 grit! I plan to finish my cowling this way, now that I have sanded off all the gelcoat (that's another story).
 
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Over two weeks later and who knows how many hours I'm almost ready to clear coat. Here's a picture I just snapped after the last 3 coats of paint. I'm going to give it a few days to cure before I use some 400 grit on the carbon fiber to get it ready for spraying the entire thing with low luster clear coat. Trying to keep it from being overly shiny.



I wouldn't call this perfect, I can find a number of places that are less than stellar and smooth, but the back area will be covered with a headliner and the door opening will have some McMaster Seals on it. In fact I used the seal material as a mold to make the fit, that's why if you zoom in you'll see it's not the prettist edge.

All in all I learned a ton for the cost of epoxy, paints and time. I feel a lot more comfortable and will be ready to tackle the outside of the top and the doors when the time comes.
 
Epoxy

I have just finished my canopy and will have my fuselage painted by the end of the week, then off to the airport. I have found after using some of the methods mention here, that West systems epoxy and their 410 microlight filler works very well and it is easy to sand.



Lyle Clarkson
RV-9A
Flying soon.
 
yep that's them. He died of cancer. I have to wonder if it was because of his life long exposure to epoxies and sanding dust? Who knows?

One interesting tidbit is that the AR-6 F1 racer had its wing built by none other than Craig Catto.
 
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