Gel coat -- does it stay or go before paint?

roee · #1 03-18-2009, 11:50 PM

So the molded composite wing tips and empennage tips come with a white gel coat on the outside. Will paint properly adhere to this stuff, or should the gel coat be sanded off before painting?

asav8tor · #2 03-19-2009, 01:53 AM

Quote:

Originally Posted by roee

So the molded composite wing tips and empennage tips come with a white gel coat on the outside. Will paint properly adhere to this stuff, or should the gel coat be sanded off before painting?

Sanded yes, off no.

I hope you are not getting ready to pull the trigger. The reason I say that, no offense, is because 90% of the paint job result will be the prep work and that question seems like you are not a paint guy, again no offense intended. If you sanded all the gel coat off you would end up with a big mess, fibers/fuzz, the part would be toast. On the translucent parts you have to fill and sand to get rid of pinholes. Both take work to get the results you want.

roee · #3 03-19-2009, 03:23 AM

Quote:

Originally Posted by asav8tor

Sanded yes, off no.

I hope you are not getting ready to pull the trigger. The reason I say that, no offense, is because 90% of the paint job result will be the prep work and that question seems like you are not a paint guy, again no offense intended. If you sanded all the gel coat off you would end up with a big mess, fibers/fuzz, the part would be toast. On the translucent parts you have to fill and sand to get rid of pinholes. Both take work to get the results you want.

No offense taken. I am certainly not a paint guy, and I have no intention nor desire to do that myself. And I'm actually nowhere near going to paint. I'm just starting to do some of the composite work (I'm not a composites guy either, yet), and so I'm just thinking ahead and trying to do the right things at this early stage.

Thanks for the info. That's what I needed to know!

roee · #4 03-19-2009, 03:39 AM

And another related question, if I may re-steer this thread slightly:

How should you prep a gel-coated surface if you need to lay up fiberglass or apply filler over it?

Thanks! (I don't mind asking stupid questions, if they get me smart answers!)

jsharkey · #5 03-19-2009, 05:46 AM

1. Clean surface with soap and water or detergent
2. Solvent clean with acetone
3. Scuff surface to a matt finish with 100 grit or finer - wet or dry
4. Wash with clean water and air dry
5. Solvent wipe with acetone
6. Check surface with a "water break test" - i.e. dribble some clean water on the surface - you shouldn't see any dropplets form like on a newly waxed car - it should try to spread over and wet the whole surface. If you do see dropplets clean and scuff some more.
7. Air dry surface
8. Solvent wipe or tack rag
9. Prime

The reason for cleaning before scuffing is so that you won't work any residual handling oil, grease, dirt or mold release into the surface when you scuff. Air drying avoids using dirty rags - although paper towels are probably OK

Jim Sharkey

If you are applying fiberglass or filler use a coarser sand paper to give some "key" to the surface ~60 grit.
Cheap latex gloves are a good idea too to avoid getting hand oils on the surface - and to keep your hands clean.

f1rocket · #6 03-19-2009, 06:20 AM

I do not believe it is appropriate to apply fiberglass cloth over gell coat. If it were me (and I built a fiberglass airplane, but I'm no expert), I would remove the gell coat down to the fiberglass, scuff the glass with 80 grit paper, and then apply any layups. Gell coat is meant to be a finishing layer that's applied to the inside of molds prior to layup. It is pretty heavy and I don't believe Van's uses gell coat any longer.

Auburntsts · #7 03-19-2009, 06:29 AM

Gel-coat is still used on the tip fairings. I asked a similar question some time back and was told to take the gel-coat off if applying additional layups or a filler like micro to build up a large area.

miyu1975 · #8 03-19-2009, 07:13 AM

um..I have been working on my emp tips (VS and Rudder done). I layed 6oz glass on top of the gel coat and aluminum to cover the crack..then filled area with Rage Extreme and sanded down nice and smooth..it looks fabuolous.. this ok I hope?

f1rocket · #9 03-19-2009, 07:39 AM

Well, since it is entirely cosmetic you are probably okay although a good whack from a hand or a passing woman's hip (at Airventure.....long story) might loosen it. I could get into a whole different discussion entirely on whether you should glass your tips on, but that would be a thread steal. I'll leave that to another thread and another time.