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True Grit

drone_pilot

Well Known Member
Okay,

I have squeeged epoxy all over my pink cowling. Now it's time to sand. My question is what is the sanding grit progression to get to a surface suitable to shoot two part primer? I.e. Start with 100 grit to 150grit, then end up at ______grit? Please help. Sanding never has been a favorite but if I knew what I was doing, it might be a little more enjoyable. 😃
 
I typically only use two grits when working on fiberglass parts.
80 grit for shaping and 160 grit for finish.
This leaves it plenty smooth for a nice FG finish in epoxy primer, especially if you plan to leave it that way for a while.
The painter will scuff sand your primer and lightly re-prime before painting anyway.
I like to use the sticky backed rolls of sand paper you can get at the auto paint shops since you can stick it on to just about anything for shape sanding.
I also use the 3 inch Velcro backed pads on an orbital sander to speed things up.
Has worked well for me over many builds.
Good luck
 
I typically only use two grits when working on fiberglass parts.
80 grit for shaping and 160 grit for finish.
This leaves it plenty smooth for a nice FG finish in epoxy primer, especially if you plan to leave it that way for a while.
The painter will scuff sand your primer and lightly re-prime before painting anyway.
I like to use the sticky backed rolls of sand paper you can get at the auto paint shops since you can stick it on to just about anything for shape sanding.
I also use the 3 inch Velcro backed pads on an orbital sander to speed things up.
Has worked well for me over many builds.
Good luck

I started the same way... finished the tail fairings and doing the pink cowls now. I ended up with essentially the same grits are quoted above.

I am using white epoxy primer so I can fly before the final paint and I found 80 or 100 grit good for a first pass, and then end up with 180 - which is the number recommended for bare steel by Kirker -

http://www.autobodytoolmart.com/kirker-enduro-prime-epoxy-primer-p-12620.aspx

If the first application of primer isn't smooth enough for you, then a 180 or 220 wet-n-dry sanding will smooth it and a second application of thinned epoxy primer leaves quite an acceptable finish. The primer has a 5 day recoat window so only the "bad areas" need to be re-sanded.
 
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How about guide coats? I sanded the first squeegeed coat of epoxy yesterday using the advice above. Problem is, I can't really tell if I'm sanding thru the epoxy. My technique was....

1.) Scuff sanded entire outside of pink cowl using 150 grit
2.) Wipe down with lots of Acetone
3.) Used cheap brush and painted on epoxy
4.) While epoxy is still wet, used a squeegee and smeared epoxy around in all directions
5.) Squeegeed off excess epoxy until cowl looked wet/glazed but did best to remove squeegee marks and pooled epoxy
6.) Let epoxy cure for 24 hours
7.) Sanded off high spots with 80 grit, then 150 grit, then 220 grit
8.) Plan to repeat steps 1-7 two more times

I can see shiney epoxy down in the pinholes but I really can't tell if I'm taking too much off. There is a place by the nostrils where I can see the fabric weave and I can feel it when I run my finger over it.

I had a guy who told me that I could fog it very lightly with a lacquer paint before applying the next epoxy coat to help me keep track of the epoxy thickness.

Please let me know your thoughts. Although I like sanding the least, this seems to be the part of the overall finished airplane that reflects a lot on the quality of the build. I'd rather not have to run and hide when folks start looking at my cowling. :D
 
The epoxy is a seal coat and pinhole filler. Do not cut through it. If you do, start again. Doesn't hurt anything, just another round of application and sanding.

Again, don't cut through it. That means no coarse grit. Like Steve said, 80 is for shaping, which is all done before the epoxy coat; at this point the 80 is already on the shelf. Use 180~220 grit to remove the squeegee marks and dull the surface, no more. Then spray epoxy primer, a high-build primer-surfacer, and block sand.

Guide coats would be used while blocking. I find it easier to spray a contrasting primer before the high-build, dark gray for example. When blocking, it shows through the light colored high-build at all the high spots. So, block until the highs are all just starting to show primer, shoot another cross coat of high-build, repeat. Might shoot a very light guide over the what you expect to be the last coat of high-build. If all the guide blocks off before any primer shows through, it's done.
 
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I'm working on the cabin top interior. Using this part to skill up a bit in fiberglass work. This thread helps but I have two probably easy questions;
1- What is meant by 'block sanding'?

2- I've not worked with high build primer before. Does any product work or is there a quality roll-on product I can use or would SEM High Build stuff work fine?
 
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1- What is meant by 'block sanding'?

Using a relatively hard sanding block to ensure that you only cut the high spots.

2- I've not worked with high build primer before. Does any product work or is there a quality roll-on product I can use?

In PPG, the usual product is K36. I guess it could be rolled on. Never tried it.
 
Thanks Dan, I'll check those paints out. I edited up my post a few mins after I posted it to include the SEM High Build stuff. I always like the SEM stuff so I might start with that.
 
You might look up "Dura-Block" on Google images.

I haven't used the product, but it gives you the idea.
I'm only a couple months behind you.
 
You might look up "Dura-Block" on Google images.

I haven't used the product, but it gives you the idea.
I'm only a couple months behind you.

Thanks, I've seen those. Wasn't quite sure what they're used for. I have some plastic blocks I can attach paper to so will try those.

I do like working with Fiberglass a bit. Fun part is if you mess up you just sand it away and try again.
 
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