The advice given on this site is based upon individual or quoted experience, yours may differ.
The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.
I pulled my boat out of fresh water after a year and cleaned the substantial marine growth off of the bottom with a power washer. There is some calcium still adhering to the bottom. Unfortunately it chipped off a few spots of gelcoat the size of a quarter. What is the best way to repair this? Can I sand/ paint the bottom? Spot repair epoxy without sanding? It's a 1995 water ballast.
Gelcoat doesn't provide any significant resistance to moisture intrusion. It's mostly there for aesthetics and to smooth over the fiberglass surface. I'd suggest you clean and lightly sand the chipped areas and fill them with a little good quality marine epoxy.
As for the rest of the bottom, whether and how much you need to sand it before you apply fresh bottom paint depends on how clean and how rough the old painted surface is. Any slight roughness creates turbulence and kills boat speed, so you'll want the bottom, keel and rudder reasonably smooth.
As an example, I had my C&C 35 hauled out every winter and pressure washed. In the spring, I washed the bottom, scrubbing it lightly with a sponge, and then painted the bottom with one fairly thin coat of a good quality ablative antifouling paint. I only sanded areas that were rough, and that wasn't much. Usually I just washed it and painted it.
After about 13 years of that practice, the bottom was smooth enough to still be very competitive in club races, but it wouldn't have been nearly good enough for high level racing.
Steve Milby C&C 35 Landfall ("Captiva Wind"); Cal 25 ("Fahrvergnügen") Past Commodore
"The size of a quarter" makes me suspicious of gelcoat blisters--I would look for more of them. (Google "gelcoat blisters".) They're more of a problem in fresh water than salt water. If you find more that haven't chipped off, that suggests (1) popping them off, letting them dry, filling the spots with epoxy (as specified in many books and West System documents), and then (2) applying an epoxy barrier coat such as Interprotect 2000 to the whole bottom to prevent blistering, and an ablative antifouling coat over that to minimize growth buildup (or at least making it easier to clean off).
Dave Bristle Association "Port Captain" for Mystic, CT PO of 1985 C-25 SR/FK #5032 Passage, ex-OUPV, Now on Eastern 27 Sarge (but still sailing when I can). Passage, Mystic, and Sarge--click to enlarge.
In line with your comments above, the answer is YES, go ahead and fill in the quarter-sized pops in your bottom gel coat. Marine Tex is okay to use below water line, or if you prefer, any reputable brand marine epoxy plus a thickener like colloidal silica should work. Make it like peanut butter and “fill them holes”. Use a plastic sheeting like Saran to cover the material, then use acetone to remove any amine blush. If you see any residual quarter-sized blisters, follow Gary B’s suggestion top burst them and protect skin, face and eyes. You may be able to ignore them if they are dime size or smaller (depending on whether you race). Afterward, as Dave Stinkpotter recommended, seal the bottom with Interlux barrier coat. I did six thin coats on Passage years ago, and the stuff really holds up well and makes the difference. I don’t sand my bottom between applications of ablative bottom paint. Each fall, during my haulout, my yard power washes the bottom to remove some green slime and grass as well as a handful of barnacles at the edges.
If you have any holes that are larger than a quarter, you should probably use small circles of fiberglass fabric stacked and epoxied in as concentric circles to build up the voids. To do this you’d pop the blister, grind out an area around the resulting hole to flatten it out, then cut 4-5 concentric circles to fill the hole. Paint the hole with epoxy, then lay in the smallest circle and paint it with epoxy, then lay in and saturate the next larger, then the next until the hole is filled. You can top the piece with Saran Wrap or Peel-Ply to protect and keep it smooth. It’s a lot of work, but for larger holes, it’s the way to go. Smaller holes can be filled with thickened epoxy. Once you finish that step, you might as well follow up with a 6 coats of Barrier Coat. You can read a lot more about this process in Don Casey’s book Sailboat Maintenance Manual
Notice: The advice given on this site is based upon individual or quoted experience, yours may differ. The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.