If the masking tape was damaged when sanding the repair area, the inner tape should be replaced. We want clean neat lines and don't want any tape strands to come off in the gel coat.
We like white hulls as they are easy to fix. Many first time kayak buyers purchase colored hulls because they look good. Once they get scratched up or damaged they don't look good and lower the resale value of the kayak. Some manufacturers use an off white. This Necky Chatham 16 has an off white hull. We could have purchased a small amount of get coat from Necky, but it would cost more then we want to spend and take too much time to get. So we mixed our own off white color.
To start off we added some yellow (bottom center container) to half an ounce of white gel coat (top center can). Even a little yellow pigment will color the white more then we wanted for the final product. We use this pale yellow (left container) to add a bit at a time to the white (right container). All we do is touch our brush to the pale yellow (getting as little on the brush tip as possible and use that to mix into the white gel coat. After mixing, we dab a bit onto the undamaged area of the kayak to test the color. Then we repeat these steps until there is enough yellow (or whatever color is needed) to get close enough to the kayak color. After testing just wipe off the test gel coat with acetone on a paper towel. Always wear gloves when working with acetone.
You are now ready to paint on the colored gel coat. If you paint on one coat thick enough to build it up to the surounding gel coat then only one coat is required. Most gel coat is laminating gel and not finish gel. Polyester gel coat will not dry if air can get to it. It needs an air barrier. The three most used options are;
1) putting plastic wrap over the gel coat,
2) sparying mold release over the gel coat
3) adding surface curing agent to the mixed gel before adding the hardener. Surface curing mixedagent is the easiest if you have it.
Clean the area with some acetone. Then when everything is ready, mix the hardener into the gel coat and paint the gel onto the area. It is better to mix onto the low end of required hardener then to add too much catalyst. A "hot" mix will dry quick, but be brittle and the repair will fail. A "cool" mix will give you more working time and will not be brittle. A repair like this, needed less then half an once of gel coat, but we mixed an once as it is easier to mesaure the hardener with a larger quanity.
Neatness is not real important. Just try to get the gel thick. Next use a plastic spreader to feather the edge as much as possible. The better the job here, the less you will need to sand, but again it is not that important as sanding a small damaged area is not too difficult.
Once you are finished painting the gel coat on, clean the brushes and containers with acetone. Then remove the inner tape before the get starts to harden. You only have five to fifteen minutes to get all of this done from when you add the hardener to the gel coat.