Finishing up Cowl Question: Pin Holes

I have not touched the gel coat on the cowl yet. I assume there are as many pin holes on the cowl as there are on the wheel pants.

I know that this is opening a can of worms, but what have people been doing to finish the cowl surface?

Does the gel coat have to come off expose pin holes?

I was thinking about trying the drywall mud method to fill holes. I tries other fillers on the pants and it was hard to get the holes to fill. Apparently drywall mud will fill on the first pass.

Follow DanH method of pinhole filling with some neat epoxy. For the thinnest coat use a metal squeegee to get it off and swipe both ways. Just pre-sand to ensure good adhesion.

I have tried it all, including drywall and would not attempt that again.

The purpose of the gel coat is to minimize pin holes.
If you sand it off, you will be generating hundreds more.

The goal with the gel coat should be to sand only enough to fully remove the gloss. A skim coat of resin or any other filler should not be needed like it was on the original cowls (without gel coat).

In places that the gel coat does get sanded through, a couple coats of high build primer is a good idea to make any pin holes visible, and to provide a sub-straight for finish sanding.

Adding to what Scott said, my painter used some filler (high build) primer to smooth out the honeycomb print-through, but pinholes weren't much of a problem (Nor were they on any of the other gelcoat parts).

Van's did a nice job of helping us by going to these parts. Not saying the same for my own fiberglass work.

I've not yet worked with one of the new gel-coated cowls, but as Scott says, a good gel coat should eliminate the classic pinhole problem. Don't sand through it unless the goal is a structural bond. Just remove the gloss and add some tooth. An epoxy skim coat should not be necessary, unless (again) you've created new glass surfaces. With gelcoat, I'd like to think a few coats of urethane high-build and some block sanding will get er' done.

DanH said:

I've not yet worked with one of the new gel-coated cowls, but as Scott says, a good gel coat should eliminate the classic pinhole problem. Don't sand through it unless the goal is a structural bond. Just remove the gloss and add some tooth. An epoxy skim coat should not be necessary, unless (again) you've created new glass surfaces. With gelcoat, I'd like to think a few coats of urethane high-build and some block sanding will get er' done.

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I just went through this on the 10 with new, gray f/g cowl. It's true that the gel coat has no pinholes. The problem is that many will develop after the epoxy primer, with it's agressive solvents goes on. Not sure why this happens. Maybe the gel coat had curing issues. Maybe it was too thin. Maybe the the gelcoat can't deal with spanning larger voids (most that appeared were much larger than the typical pinholes). Maybe I blocked too much off trying to flatten out the honecomb pattern that was telegraphed through the gel coat. In hindsight, I should have shot a bunch of 2K on it to get rid of the telegraphed honeycomb. However, limited block sanding will leave hundreds of honeycomb shaped spots still unsanded and glossy, unless you do it by hand, without a block, and accept the poor results.

Be warned and definately follow the advice about limited removal.

I ended up using 4 coats of epoxy prime and let it cure for a couple of days, then 2-4 coats of 2k primer. Then blocked the **** out of it to remove most of those layers. It worked.

The new gel coated parts are not what I would call a successfull fix.

Larry

I was not going to sand down the gel coat too much, but the wheel pants were a nightmare. I still don't think that they are finished, but close enough.

I had to work with one of the old (pink-ish) cowlings......no Gel-Coat that is.
I tried DanH’s way followed by epoxy 2k spray-on filler.
Sanded, fillered, sanded, fillered, sanded, fillered, sanded and sanded some more, and so on......
The dust was everywhere.............EVERYWHERE!!!

IMHO, to get a good result and no pinholes it doesn’t matter which way you go, which product you use.....
It only takes elbow-grease, sh*#tloads of elbow-grease,filler and sandpaper.

But when you are there, when you’ve succeded........it’s priceless.

Welcome to homebuilding!

All composite parts in kits for all RV models that could have gel coat incorporated into the production process have had that done (unless it made no sense to, like induction airboxes, etc.)

That being said, it is not the same gel coat on all parts.

On pre-preg parts like the cowls and RV-10 cabin top, a dry film coating is used. It is sheets of film that get layered into the mold before the pre-preg material is put in place.
It slightly liquefies when the pre-preg is oven cured. Because pre-preg by design is very lean on resin, the fabric weave doesn't fully fill with resin like it does doing wet lay-up molding and the dry film does get drawn into the weave texture slightly which does require a bit of filling / finish sanding. Usually a couple of spray coats of high build primer followed by some finish sanding is enough to make it paint ready. It is much easier than what the pre-preg parts prior to the gel coat film were.

All of the non-prepreg parts use a sprayed in gelcoat which is generally pretty smooth on the finished parts but there is usually still a small amount of weave print through because of post cure shrinkage. It is usually slight enough to just lightly sand until the texture is gone and then do the normal process used for painting. This is what is done for our SLSA production and the parts come out looking fantastic.