Pro clue about gelcoat-crack repair
I use the little stone tool in the Dremel, rather than the carbide grinder bitt. I find it gentler and more controllable. Also it is cheaper. And it lasts as long as you'd care to use it-- I've done about four boats with the current one and it's still fine.
I opted to forego the Dremel-brand machine, especially the battery-powered one, after seeing several coworkers cuss theirs out when the batteries died too soon or the switches gave out. My 'Dremel' is a Blue Hawk 115vac one from Lowe's. It's easily used and the speed control is reliable and clearly marked. This has been what coworkers have asked to borrow when their Dremel-brand ones failed them.
One 'pro' trick: when you are tracing the crack with the grinding tool, make one full pass over some smallish area first. Then, wipe aside the dust and inspect the gouge you just made. If you see any dark line inside it, you have not gone deeply enough-- the crack is still there. Don't worry about going down to the 'glass-- this may be preferable in most cases. Also, if you see any gaps at all, voids, cracks, chipping, etc., where you have gouged, dig it all out. This may prove alarmingly dramatic; but it's necessary. Chase that damage till there's no damage left. And there's no sense in going past a few 'small' ones. Any of them can reveal problems-- pursue them all. This job, if done right, can easily take up more than a weekend.
I filled some of my topside cracks with Evercoat 27; but the cream hardener failed in the most recent application, the stuff never kicked, and three days later I had to remove it all and refill the cracks. After that I used WEST epoxy with 403 filler, same as I used on the bottom. So long as it's not a wide, deep area (which gouges won't be), epoxy will not shrink or crack from the expanse and is really the best thing to use.
If it's a larger area, or if there are so many cracks that you might as well just trowel on a whole layer (particularly underwater), consider MAS milled fibers. This is not the cheapest way to go; but the milled fibers are in fact the same material as the fiberglass, so they will add the most strength with the least work (as compared to poking 'monkey hair' into the gouges by brush/stick/finger/hand/face/etc). Milled fibers are just the very devil to sand, so go thinly and evenly from the start.
All of this can be finished with a skimming with gelcoat; but if you add any filler to the gelcoat keep in mind that it will change color. Your best bet is to leave it a little rough (100 grit?) and fair before applying several healthy coats of gelcoat by itself, enough to permit you to fair it with sandpaper (120/180/220/320/400/600+). These can be done with a roller-- spraying it won't give you enough buildup. Going too thin with the gelcoat will let you burn through to the repair, over and over. Just don't put on the gelcoat so thick that it'll crack.
Interlux recommends trowelling a thin mix of epoxy with gouge filler over the whole bottom before application of Interprotect. Fortunately I 'accidentally' did this before opening the can (and then reading the guide). In any case a two-part epoxy barrier coat is de rigueur after filling gelcoat cracks, especially of any of them were sort of deep.
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