[Hope my advice here is not too basic. I wrote it so a future reader may know more about what to do. You may find much of it redundent with what you know already.]
I just fixed a big gash in my hull. It was made by my home-made winter cover last year. The gash was 2"x1" and down through the gel coat, so maybe 1/4 inch deep. It was easy to apply the gel coat and easy to sand. That's as far as I got (today).
I went online and ordered Spectrum Color gel coat in the matching color for my boat. Go to Spectrum Color
. You specify the manufacturer & year and get a 2-part gel coat that is EASY to use and matches the color.
Go to Ace Hardware (or other hardware store) and get the variety pack of wet sand paper. It has 220, 400, 500 grit. Also get some 60, which is not in the variety pack. You'll want to apply extra gel coat and then sand down to the level you need. The 60 helps you quickly get down to near where you should be.
Get plastic kitchen gloves, get acetone. Get a cup to mix in and something to mix with (I got those 2 things from West Marine).
The two part gel coat has instructions. There's a little glass jar with a creamy (somewhat dry) white past and a little (1/ 2 ounce) dropper of catalyst. The catalyst is the stinkiest thing you'll ever buy, and the packaging has warnings all over it to make sure you use it outdoors. After using it, I had to wrap electrical tape around the lids of both containers to keep the stink out of the boat. (And then I put it in a ziplock bag for added protection.)
Rough up the area with your 60 grit sand paper. I used a wire brush (looks like a toothbrish but with metal bristles. I read somewhere about not using a steel brush because little particles of steel get into the gel coat and make rust spots. Of course, I didn't remember that until now.
Wipe down the area thoroughly. I used acetone and paper towels. Wear the gloves, acetone is nasty.
The instruction say to use 1/2 the paste and (if I recall correctly) 3 drops of catalyst. I suggest making a smaller batch than that. It sets up very quickly -- faster even than epoxy -- and working in small quantities is much easier, especially until you get the hang of it. One minute you are spreading soupy white stuff, the next you are spreading gravel. Besides, using a 2" putty knife, the first coat dries with a rough texture. That roughness is perfect for applying the second coat, which fills in the roughness and makes for a thick covering. Ditto for a 3rd coat, so make small batches. (I quickly wasted one container by doing the opposite of this advice, so I know.)
I used the metal handle on a west marine mini epoxy brush for mixing-in the catalyst. There's so little catalyst used, that I'd suggest putting the drops directly onto the hunk of pasty white stuff, placed on the inside rim of a plactic mixing cup. The drop sort of gets absorbed and doesn't slide down to the bottom of the cup. Remember we are talking small amounts here. By tilting the cup I managed to do all the stirring on the side of the cup, which let me get all the mixture out with my putty knife. (Then for the next coat, I flipped the cup around and used the other side (still the inside of the cup, 180 degrees around from before).
Spread with 2" putty knife. It sets up fast. (I know I said that already.)
Clean up with acetone.
Wait until dry. The package says 2 hours. (I waited a week due to my schedule.)
Sand with finer and finer sand paper. Scotty's gave good advice about feathering. I just sanded until it was about right. My block (with sandpaper wrapped around it) naturally sanded areas adjacent to the (rough) repair, so I got a lot of feathering naturally.
Now at the end of this day it looks really
good. It's difficult to tell where the gash was. I mostly know because that area is so clean and the rest of the hull needs a good wash. I still need to fill in some very tiny spots where I didn't apply enough of the mixture. They are so small I'm debating whether I should wait until next year.
Anyway, hope that helps. It was much easier than I thought it would be.