How to prepare bare fiberglass for bottom paint

[mike66]

Some underwater areas have cracked and peeling gel coat. I ame able to remove the old gel coat with a scraper in these areas. There may have been impact at some time, since most of it is near the mid ship chines. My question is, would you sand the area and adjacent gel coat, then prime and paint, or would you fair in the area with with epoxy or polyester first before the primer. There are no blisters on the bottom. The areas are about 1X2 feet.
Thanks, any suggestions besides peeling the entire bottom are welcome. [emoji854]

 

[DDW]

I think the standard repair (assuming no laminate damage) would be to sand it and re-gelcoat. It can be brushed on, just like thick paint. Then scuff, barrier coat, and bottom paint.

 

[SoWhat]

Normally the standard repair would be adequate but properly applied and cured gelcoat can't be removed with a scraper so the question becomes is this a poorly done repair? I would be tempted to go down to matt and do some sort of dye test to make sure there are no cracks. Maybe some thinned bottom paint and then sand it off. Once you are down to matt the proper repair would be fair and paint.

 

[Larry M]

I'd fair the area with epoxy using something similar to West System 407. Prep the area first to get to clean fiber glass/gelcoat. If it's just the gelcoat that you have lost (~5mm thick), wet the area with unthickened epoxy then fair with the 407. You'll probably need to use a long board to fair the repair to match the existing gelcoat. Then use a barrier coat and bottom paint. If the glass mat has been damaged do to impact I'd talk to a pro.

 

[Comodave]

I would remove any gelcoat that is suspect. Then use Interprotect barrier coat on the whole bottom. Check Interlux’s web site and see how much paint they recommend for your boat. Then put on at least that much of the barrier coat, keep painting coats on until you use all the paint up. I use 1 more gallon than recommended so I will get a good thickness of the barrier coat. You paint a coat and wait until it is thumbprint dry, you stick your finger on the paint and it will leave a fingerprint but not transfer to your finger. Then do another coat. You don’t want to wait too long or you will have to sand between coats. It will probably be 4 to 6 coats of barrier coat paint. Then do the bottom paint when the last coat is thumbprint dry. Use Interlux bottom paint with the Interprotect barrier coat.

 

[Eagle Pursuit]

I agree with SoWhat... this probably should not be a “Cover and Paint” repair. If you suspect impact damage then strip it down and do a thorough inspection and then decide the next course of action.

 

[STB]

My own perspective is that I'd want to get down to good substrate, both in terms of area and depth. I'd take off as much as wants to come off.

As for filling the area, if above the water line and it could be built up with a couple or three coats of a good high-build primer, I'd do that. But, that doesn't sound like the case.

If the original thickness of gelcoat would do it, I'd consider that an option. But, thick gelcoat tends to craze, so I wouldnt be tempted to cheat, even a little.

Personally, if it is beyond just primer, I am a big fan of thickened epoxy, using the right thickener for the depth of fill, using a fairing filler for a top coat, as needed. My view is that epoxy is strong, flexible, adheres well, and protects the glass. And, with the right thickener, it sands like a dream.

Just make sure to get the amine blush totally sanded off the surface before priming and painting. And, beneath the water line, to avoid blistering, use many coats of primer. 5 or 6 or so (vs 1 or 2 or 3). Go past the damage to feather. The primer, if thick enough, provides a lot of protection from blistering.

My own favorite primer is Interlux Primekote.

 

[Comodave]

Typically amine blush is best washed off with water. Sanding it will gum up the sandpaper very quickly.

 

[mike66]

Thanks for your ides. It appears that the underlying mat is intact and solid. I've had cracked and peeling gel coat on other areas of the boat, mostly on the flybrige eyebrow and sides, perhaps through water intrusion and freezing. There I have removed as much loose stuff as possible and faired with either thickened epoxy or polyester (bondo).