Dumb question about sanding and gel coat

[cyrano138]

This is the bottom of a 1974 cal20. I'm sanding it so that I can do an epoxy barrier coat and get the keel back on, and I assumed I knew what I was looking at, but now I have two questions.

1. Is the white stuff the gel coat, or is it something else? There is a layer of very very thin blue underneath it and then immediately you get to the chopped strand mat of the hull itself. I'm asking because I have to at least get down to the gel coat to put on the epoxy barrier coat, and I want to make sure this isn't some kind of weird paint (and maybe the blue is the gel coat?) or some nonsense like that.

2. In the same photo you can see where there are cracks in the white stuff (very small) that, when I sand them down, reveal what looks like tiny blisters. But it's just the gel coat that seems cracked or defective. I don't see anything wrong with the resin underneath.

[er9]

the white layer looks like your gelcoat. the blue layer was maybe an added layer on top of the gelcoat (behind) to help with wetting out the glass.

[cyrano138]

Are the little defects likely to be blisters?

[er9]

no they dont look like blisters. hard to say what happened there. possibly sloppy gel coat application and the blue resin may have been applied to fill in all the voids.

[cyrano138]

It seems to run alongside the keel. I wonder if the hull flexes there too much and it's stressed the gel coat.

[er9]

Originally Posted by cyrano138

It seems to run alongside the keel. I wonder if the hull flexes there too much and it's stressed the gel coat.

Ah. it could have been overcoated at some point in time from a previous owner if its around the keel joint. maybe the blue was a prime coat. anyones guess without knowing more about its history or lots more photos. stresses usually manifest as cracks not bubbles. the pictures dont look like stressed fiberglass.

[Benz]

There are always weird sights like that at the intersection of gelcoat and laminate. Those spots could be drips of resin that fell onto the gelcoat in the mold before they laminated; the blue could be a tie coat of some sort (one boat I had from 1967 had a similar black layer) to prevent print-though on the gelcoat or keep light from seeping through. As long as there are no air bubbles or voids, that white surface is probably fine.

[Orion Jim]

Sand the gelcoat down to the fiberglass in another area, just a small test spot, to see if you find the same thing. If so, don’t worry about it. They just had some unusual molding technique which has proven itself capable over the last 46 years.

[cyrano138]

It seems to be present to a small extent all over, but is much more pronounced (as in the photo) running fore to aft on either side of the keel about six inches out from it. The glass underneath doesn't look cracked or deformed or discolored. Hopefully it's okay. I'm going to replace all the bulkheads and l, when I do that, I might lay up a few extra layers of glass on the inside for some added strength.

[cyrano138]

Originally Posted by Benz

There are always weird sights like that at the intersection of gelcoat and laminate. Those spots could be drips of resin that fell onto the gelcoat in the mold before they laminated; the blue could be a tie coat of some sort (one boat I had from 1967 had a similar black layer) to prevent print-though on the gelcoat or keep light from seeping through. As long as there are no air bubbles or voids, that white surface is probably fine.

Fingers crossed 🤞

[Spot]

Originally Posted by cyrano138

It seems to be present to a small extent all over, but is much more pronounced (as in the photo) running fore to aft on either side of the keel about six inches out from it. The glass underneath doesn't look cracked or deformed or discolored. Hopefully it's okay. I'm going to replace all the bulkheads and l, when I do that, I might lay up a few extra layers of glass on the inside for some added strength.

Great little series from Sail magazine and West Epoxy on Youtube, it may have been mentioned recently, I watched them all this morning. Very much aimed at their products but they make a nice statement that their techniques work for any 'high quality epoxy'. They do dings, fix rotten core (my next big problem), bulkhead replacement, and bomber hardware installs on a J24. They also cover amine blush and solvents.

https://www.youtube.com/playlist?lis...zq_pqstU-0Yefi

[coastalexplorer]

It is common practice after spraying the outer colour gel coat. to spray a contrasting colour coat before commencing layup. The concept of the contrasting colour being as a marker to warn when sanding or polishing the hull when the outer colour gel coat is becoming too thin.

[Singularity]

My take is that this is completely normal, no damage, don't worry about anything. The gelcoat is simply checked and as stated above the blue coat is the interface between the gelcoat and glass. What looks like spots is merely where the raised/proud bits of the checked gelcoat get sanded away first and, because the gelcoat had already raised off the blue layer, a spot quickly forms as the sandpaper "picks at" the proud gelcoat.

[cyrano138]

I appreciate everyone's responses. I'll post a few more photos tomorrow of the whole area around the keel sanded. It doesn't seem to follow any real pattern (sort of). In any case the glass underneath it seems normal so I feel alright putting the epoxy barrier coat over it and being done with this neverending project.

At this point if I tried to make it all perfect I'd be dead long before wit ever touched water. Hopefully I don't sink halfway between here and puerto rico.

[jhulmer]

Your gelcoat has worn thin and you are into the rough barrier between gelcoat and fiberglass. The blue is a thin coat of gelcoat colored to block light from shining through the hull. This is common practice although usually it is black.