I have noticed (well, might be that it is a usual way to use it but I just haven't heard of it) that Teknodur polyurethane paint that can be used to paint composite structures like those on experimental aircraft, can be applied with brush and then perfected with sanding like on applying topcoat (/gelcoat) on a sailplane.
This just works for me, please do not follow if you are not willing to take the responsibility of potentially ruining your paint:
0. Do not use base paint or raw epoxy method, you don't need to fill pinholes, just forget about pinholes with this method! In other words, you can directly apply like this on top of smooth sanded dry micro or automotive polyester filler!
1. Apply thick layer of Teknodur 2 component polyurethane paint (e.g. white) on top of the composite structure. Any other similar polyurethane paint works too (I have also tested with Hempel 2-component boat polyurethane paint). Base paint is not necessary, the Teknodur takes on a bare epoxy surface which is sanded to dull (be sure it is sanded to dull, if it is not, then it will not take, but peels off). Do not use solvent to make the paint thinner, the thick property is desirable. The thick paint blocks the pinholes on the surface below.
2. Let it cure and then inspect. Look, 1 layer of paint and no pinholes! There may be runs, but you can get rid of the runs easily!
3. Wet sand the surface smooth. Use quite coarse grit at this point.
4. Add second layer of Teknodur paint. You can use a bit solvent now, and you will get no pinholes. Try to avoid runs more carefully at this time.
5. Wet sand to completely smooth finish.
Use all available wet sand paper grits up to 2000 if you can find 2000 grit. 1200 grit is fine though.
6. Use polishing compounds to finish the surface to high gloss.
7. Add vax and polish.
A little bit tedious with all the wet sanding, but on the other hand: full control over pinholes, no base needed, and most sanding goes to the paint without harming the critical glass/carbon fabric under it.
I am just in middle of painting a little composite part this way and I have noticed that it works. Before you ruin any large parts by using a method where the paint is misused and done differently than all painters will teach you, please try it to some scrap part first. I have finished two scrap parts like this and they have been in the snow and ice the whole winter without any harm done to the paint surface, so I would guess that this sanding method does not ruin the paint.
I am not sure, but it could be that:
- You would be even better off if you first apply a very thin layer of paint that enters the pinholes. Sand dull. Then don't care about the pinholes, just add the thick layer of paint on top of the thin layer.
On the base and on the first layer, the sanding result does not need to be smoother than 240 grit. Anything more than that is waste of time because the thick paint rounds the minor irregularities.
Pros:
- Polyurethane paint is easy to sand, very very very very easy compared to sanding epoxy
- Runs on polyurethane paint is no big deal, just sand them off in a minute and you are done!
- Quick to finish
- The thick paint is very weather resistant and is as smooth as you sand it
Cons:
- The layer of paint becomes pretty thick and it is heavy, and in some cases might be undesirable.
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