You are trying for a "mechanical" bond. If you shoot multiple layers of gel coat, you want to do it before the first layer flashes off so that a chemical bond is achieved for the those layers.
This is true if your using Duratec. Treat it like paint. However, If your shooting gel coat without wax or Duratec you can let each coat kick before the next coat and still have a chemical bond. Gel coat without the additives stays sticky after curing. Then mix the Duratec or wax in the last coat.
I've quit using Duratec. The cost isn't worth the slightly less orange peel ( still have to sand and polish )
Block sand with 80 to start. Fill any bad spots. Sand those areas till smooth. One more time with 180 all over. Spray two to three coats w/out additives. One last coat with additives. Then I would block with 180 until the peel is almost gone then wet sand 220, 400, 600, 800 and buff. Hopefully you don't go through.
It's the bottom of a ski so I wouldn't put too much into it.
When your spraying make sure your careful when feathering and don't stop in the same place as you put coats on. You can wind up with a few very high spots.
