Sanded, filled, epoxy wiped---now need some advice

Hey composites wizards,

It been long hours around here working on the cabin top. So far I've sanded down the bumps, applied micro, sanded that down, and done three epoxy wipes.

The problem is that there are now small ridges/waves in the epoxy surface. Perhaps I'm a bad squeegee-er or maybe this is to be expected. The ridges are hard to photograph, but they look like linear streaks in the surface---a fatter version of the lines left by a paint brush.

I've tried sanding with 180 grit and a .032" aluminum backing plate. This roughs up the surface but the troughs of the waves are still visible.

What's the right way to solve this one? More sanding? High build primer?

If sanding, what grit would you recommend? I'm a little concerned about sanding through the epoxy shell, but I don't really know much oomph that would take.

DanH made this look so easy...

David

IF I understand your problem correctly, the sanding block may be following the surface rather than sanding it smoothly. I use, among may blocks that I have, a 3/4 in/ 3in / 12in piece of oak to start with. anything that is stiff enough so not to follow the high and low of the surface. and while not applying to much down pressure. We have several commercial sanding blocks but I seem to mostly start out with the wooden one
Glue a piece of sand paper onto a solid block and try it?
Just an Idea. Larry

David removed the hood , grind it, take the number 100, then take the finish plaster covered the hood and it too is polished, and then apply gun from the ground in 3-4 layers, let it dry and grind it, about 600, I'm sure you get. This process for about 5 days.
Maxim

Post some photos David, let's see what you have.

Waiting in the gallery to see what Dan says. More of us out there struggle with this....

Here's a photo that captures some of the "lines":



The angle exaggerates the roughness, but I think you can get the general idea. The axis of the waves is roughly north/south in this image.

Thanks for the help!
David

One photo?

Sorry-didn't think it would take take more than one to capture the gist.

Here's the only other one that I took this morning:


David

Sanding. Then More Sanding. After that, sanding.

Ok, having done this process for an entire aircraft, maybe I can help.

First, to what level did you sand this part before you did the epoxy wipes? 180? With a block?

If I was doing this, I would have started at ~36 grit, and have gone all the way down to either 180 or 240 grit. This has to be done with a block, not your hand. I highly recommend the Dura-Block kits. You can get them from Northern Tool for about $55. For much larger parts (wings, fuselage) the 36" 3M fairing boards are excellent.

Sand with the axis of the sanding block in line with the axis of the part, but push the block 45 degrees to the axis. You have to go past twice to go both 45 directions. When you think you are done at 180 or 240 grit, spray it with black guide coat and sand again at 180 or 240. The part you thought was faired in is probably not, so keep sanding/guide coating until it is.

That brings us to where you are now, which is that you have your epoxy wipes and lots of waves. The answer is simply to keep sanding. Same process, axis of the block aligned with the axis of the part, 45 degrees, both directions. When I do a WEST wipe I tend to start at 80 or 100 grit, as you need to break the surface. Quit that as soon as you are through (two passes usually, no more), then progress on through 120, 180, and 240. Don't bother going past there, as the primer wants something to hang on to. Guide coat this again, sand it out, and the waves should be gone. If they are not, I would suggest to you that you did not have them out when you were at the filler stage.

Also, the epoxy wipe provides an opportunity that most people miss, which is that when it is wet out you can use the fluorescent lights to highlight any waves or dips in the part. When you are at the wipe stage you should not have any.

Here's an example on my wheel pant mold. You can see the lines from the lights in the wet WEST are nice and ripple-free. The underlying water lines are caused by the MDF the mold is made out of. Also, this is a HIGHLY compound curved part, and life is much easier on parts with curvature in a single direction.

First of all, thanks to everyone who has made suggestions thus far. They're a big help for this composite noobie.

FYI---These results are my best effort as following the "Cory Bird" process which has you block sand with 36 grit before going to the skim coats.
http://www.ez.org/pages/waynehicks/chapter_25_skimcoating.htm

It sounds like the prevailing theme is that I need to sand more (with a block). I'll get some 80-100 grit and attack the surface before going back with finer grades and then on to Ekofill.

David

What Kevin said.

This isn't composite work as much as it is "Basic Body Work 101". It's about cutting down the high spots and filling the low spots, at every layer, before going to the next layer.

The only thing I'd add is that you don't want to sand through the epoxy coat into the fiber glass. Do that and you open new pinholes; you'll need to re-skim.

David, in an earlier life I used to rebuild wooden boats at Lake Tahoe. One tip I can give is to buy 4 inch sanding belts and tear them to the size you need. The sanding belts last much longer on the hard west system epoxy. You can use spray adhesive to stick them to blocks of various sizes. Good luck

Go buy a can of cheap sandable primer from autozone. Spray it on the piece then sand it all off, then it will be level. Or look at it and it will show you the high and low spots. Just use it as a guide coat. I wore my wrists and arms out sanding! For a year, I couldn't walk into Walmart or Lowe's without going to the sandpaper section and restocking.

digidocs said:

First of all, thanks to everyone who has made suggestions thus far. They're a big help for this composite noobie.

FYI---These results are my best effort as following the "Cory Bird" process which has you block sand with 36 grit before going to the skim coats.
http://www.ez.org/pages/waynehicks/chapter_25_skimcoating.htm

It sounds like the prevailing theme is that I need to sand more (with a block). I'll get some 80-100 grit and attack the surface before going back with finer grades and then on to Ekofill.

David

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First, I think DanH thinned the epoxy coat and rolled it on. I bet his glossed out (glassy looking). If you use a squeegee then you will get waves etc all over the place. It is not you, just the process. Even if you think it is micron thick. I just did this yesterday on some pinholes over nicely sanded epoxy primer. I wet sanded with 220 (wet or dry paper) under a heavy rubber pad (like 1/4" thick conveyer belt) until the light reflection was smooth. It follows slight contours, but cuts down the peaks.

2nd, I have found that if you want to sand resin dry, use the green, open coat 80 grit long board paper. It is tough, it won't clog and lasts longer than your hand will. Or use a 3/32" orbital with same 80 grit.

Lots of sanding is the answer, I would not add yet more filler. (until this is down, but not through the epoxy)

Curious, why did you add the layer of epoxy? Pinhole filler?

Thirty years ago Burt Rutan showed us how to do this. He never applied epoxy over an area that needed contouring. Epoxy cures hard and is difficult to sand. The base filler material should be as dry as possible to facilitate sanding, not coated with epoxy. After the contour is established with relatively soft filler material, glass is applied to provide strength, the number of plies depending on the structural needs of the area. Cured epoxy is smooth and almost nothing will adhere to it without roughing it up or using peel ply.

The windscreen requires applying glass first to attach it to the top skin. Then the area is filled and sanded to contour. Its a messy job but is easier by not applying epoxy over the filler material. The instructions combine applying glass and filling and contouring in the same steps but it works better to do the glass first and then contour, or the other way around if you choose. But not at the same time.

I am no expert working with composit materials because I hate the process with a passion, but I have built a couple glass airplanes and know how it should be done. The Van's instructions are not clear in this matter. Burt Rutan's instructions were better written and easier to follow. But it still was messy work.

PS Epoxy should never be used as a filler. Its only function is to keep the glass or filler material in place.

David-aviator said:

PS Epoxy should never be used as a filler. Its only function is to keep the glass or filler material in place.

Click to expand...

Are you saying should not be used for filler period..? The previously posted link makes sense to me - skim coat vs micro for pinholes. Micro only bridges pinholes, does not fill.

http://www.ez.org/pages/waynehicks/chapter_25_skimcoating.htm

I've yet to touch any glass work, so I'm definitely not questioning your knowledge, just surprised reading this comment. I'm currently reading Rutan's book and reviewing a lot of posts here getting ready to start some glass projects. Enjoying the knowledge in this thread

60av8tor said:

Are you saying should not be used for filler period..? The previously posted link makes sense to me - skim coat vs micro for pinholes. Micro only bridges pinholes, does not fill.

http://www.ez.org/pages/waynehicks/chapter_25_skimcoating.htm

I've yet to touch any glass work, so I'm definitely not questioning your knowledge, just surprised reading this comment. I'm currently reading Rutan's book and reviewing a lot of posts here getting ready to start some glass projects. Enjoying the knowledge in this thread

Click to expand...

Rutan never thinned epoxy. Pin holes were and are a pain in the butt and we filled them with spotting putty. There are better products on the market today, the auto paint guy who did my airplane used some stuff he brushed on and was an easy sanding job taking it down, don't recall what it was but an auto paint store would have it, it is quite common.

Rutan's take on epoxy was use as little as possible, it only adds weight to the airplane. Lay ups should not be dry but just enough epoxy to wet out glass. Squeegy, squeegee and squeegee some more. He said its always good to have lots of epoxy on the work shop floor - not on the airplane.

His methods worked but today most people including Van's do it differently.

Go buy a can of cheap sandable primer from autozone. Spray it on the piece then sand it all off, then it will be level. Or look at it and it will show you the high and low spots. Just use it as a guide coat.

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Using a guide coat is an old school approach for working in a dim, dark shop. With bright florescent lighting, highs and lows are easy to see by looking across the part at a low angle, or by shine.

Shiny low spot, upper middle:



See the little wave on the right?

For a year, I couldn't walk into Walmart or Lowe's without going to the sandpaper section and restocking.

Click to expand...

I'm a huge believer in pro grade abrasives, cost be damned, the minimum being 3M Stickit Gold, or Green Corps papers.

First, I think DanH thinned the epoxy coat and rolled it on. I bet his glossed out (glassy looking). If you use a squeegee then you will get waves etc all over the place.

Click to expand...

I never thin epoxy. That's ancient stuff that just won't go away. Don't believe it? Mix some epoxy, pour off half into a separate cup, thin one significantly, leave the other alone. Come back tomorrow and give them both a scratch test, maybe break them...assuming the thinned sample even cured.

If you need a low viscosity, skip the ad hoc chemistry and buy a low viscosity. I've experimented with standard and water-thin, and there's not much difference in the end result. I tend to use a standard laminating viscosity for large parts and a single painted-on coat of thin for little parts, just because squeegee work on small, curved parts is a PITA.

In recent years I have started using a roller to even out an epoxy coat, and eliminate squeegee marks. The resulting cured surface looks like orange peel and has no ridges, which is really good...it's a terrific sanding guide. You can accurately sand the surface slick without breaking through, assuming of course than the underlying surface was already flat.

I just did this yesterday on some pinholes over nicely sanded epoxy primer.

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Don't think I'd epoxy skim coat over primer. The goal is a slick, firm, sealed surface, pinholes and voids filled, before starting with any primer or paint.

Thirty years ago Burt Rutan showed us how to do this. He never applied epoxy over an area that needed contouring.

Click to expand...

Correct, in that all contouring is done prior to epoxy sealing and primer.

1. Fabricate part. Extra time spent on quality forms and molds pays back later in less finishing time, less filler, and less weight.

2. Surface fill as necessary using dry micro and sand to exact contour. Here the pencil is marking places which need more attention.



Epoxy skim and sand slick.



Epoxy primer, followed (when desired) by urethane high build primer and blocking.



Then paint, cut and buff as desired.

Dan Horton is a pro at this stuff, follow is lead and the results will be pleasing.

DanH said:

I'm a huge believer in pro grade abrasives, cost be damned, the minimum being 3M Stickit Gold, or Green Corps papers.

I never thin epoxy. That's ancient stuff that just won't go away. Don't believe it? Mix some epoxy, pour off half into a separate cup, thin one significantly, leave the other alone. Come back tomorrow and give them both a scratch test, maybe break them...assuming the thinned sample even cured.


Don't think I'd epoxy skim coat over primer. The goal is a slick, firm, sealed surface, pinholes and voids filled, before starting with any paint.

.

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Papers - +1

Thinning, I said I "think" Thanks for clarification.

Over primer - yeah , not ideal, just to kill off the last pinholes, did not want to use filler. I just sanded (wet) back down to the primer as the guide, and poof, pin holes gone. The real point was that a squeege leaves marks and sanding takes them out. Pinholes are really hard to see with a clear substrate, I had no filler as this was a part from my shiny mold.

Keep us straight Dan !

Thank you Dave. I'm just a homebuilder like you.

I did spend my OSH with some real pros this year. That's a spread weave carbon/Innegra shell infused with urethane methacrylate resin. Look for more in a future issue of Kitplanes.

Where is Dan in that crowd?