I will bow to all of that, and good advice. Maybe what I should have said instead of fiberglass not being waterproof, is that the matting in the layup I have seen from my own dings, has cloth exposed, so that would certainly wick to a certain degree wouldn't it
@Glassman ? Secondly, if a below waterline gouge is left unattended, or is not completely or correctly repaired, it can delaminate can't it? Beyond that, I am not the fiberglass/gelcoat expert and I recognize that you are, but I want the boat to look as good on the trailer as it does in the water, so my expectation of "looks" might be different than yours. I have filled my own small and out of the way dings with marinetex and will in the future, but I don't think I would like doing that on a larger area. But again, your wisdom on this may change my thinking, because I just assumed that repair with dissimilar materials would not be as capable as would a gelcoat repair, even under a keel guard. It also is a question of if you intend to do this repair yourself or hire an expert, isn't it? I mean if you took a boat into the shop, you wouldn't do a marinetex repair on a foot long area of damage would you? If you would, you really are changing the way I would look at it!
OK, I'll try and tackle these as you wrote them...
Fiberglass, as I mentioned, is inert. It isn't waterproof per se, it's glass fibers that can be made into a cloth-like fabric (many different weaves, thicknesses, directions, etc.), roving (think rope or strands) chopped strands...on and on. This is basically the material that adds structure/strength when it is wetted out with the resin and allowed to cure. The resin will envelope the glass fibers and form a skin when it hardens. If the resin barrier is abraded, scratched, gouged, or otherwise pierced it may allow water to enter in that area. If there are multiple layers - when water penetrates it will wick and have a tendency to travel and that's where your "delamination" begins. Blisters form and the substrate swells. If left unrepaired it will only get worse. If caught immediately it will usually be localized and easier to repair.
My expectations of "looks" are probably identical to yours. As I mentioned, I always prefer to repair gel coat with gel coat where it is visible. If the damage is one inch outside the coverage area of the KeelGuard I would repair that with gel coat.
If the damage is
all inside the coverage area of a guard I wouldn't go to the extent of making a perfect gel coat repair - polishing it out to look like new, if I'm only going to cover it with a guard.
Too much work for not much in return in my book. I'd personally use West System epoxy. (if large, use their micro balloon filler material) Let it cure, sand it smooth and apply the guard with 5200.
It will look no different than a brand new boat with a guard applied. In my opinion the epoxy is superior to the resin used in the construction of these boats. It's fairly costly and that's why you don't find Yamaha using it in their boats.
Marine-Tex is an epoxy for patching small scrapes and gouges. Not the best stuff for a foot long area, not even if you mixed in chopped strand (which I have seen done with mixed results). Think 1 or 2 inches or smaller typically. Maybe a bit bigger if it's right on centerline and not deep enough to affect structural rigidity AND it's going to be covered by a guard using 5200. Then it would work well.
Anyone with any reservations about doing a repair of this type, especially below waterline, should seek out an expert repair shop for advice first.
As to what shops do....I have seen repairs done with resin, chopped strand and newspaper.
Some are actually very good at painting an entire boat instead of repairing large areas with gel coat. The new two-part paints are amazing.
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