Gelcoat is never thickened with fillers. Are you talking about making a fairing compound out of gelcoat using fillers? If so, I have done this. It isn't as structurally strong, but for fairing this isn't usually an issue. The advantage is that you get a fairing that is closer to the final color, so it doesn't take as many coats, and any sanding that gets close to breakthrough doesn't show as badly.
You don't want either of those fillers for fairing, as they will turn hard as concrete. Use microballoons to make a softer more sandable fairing.
I do sand the filled area slightly down from the surrounding area, with a taper into the surrounding area. The trick is "SLIGHTLY". If you make a divot, it will look filled when covering it, but you will see it noticeably when it is polished up and the halo around it will be worse. One way to make this more fool-proof is to brush a thick layer of gelcoat into the sanded down area, let it cure, then sand it flat. Then spray gelcoat for the finish. Yes, you will likely sand most of the sprayed gelcoat off, but any imperfections will be filled and it won't have any burn through.
Don't chase perfection here. You almost certainly will see the new/old gelcoat interface halo and be tempted to sand further out to remove it. This will increase your chance of break through, and send you chasing a larger and larger repair.
If you have filled the gouges, you don't need to worry about gelcoat thickness when spraying. You won't spray thick enough to matter. Just spray so that the middle of the repair is higher than the surround, and the edges feather out slightly. It will kick a bit faster when thick, but for the thickness you will be using, this won't significantly change your work time.
Sunlight and heat do effect cure time, but you adjust for this with the ratio of catalyst added. If it is hot and sunny, just stay on the lower 1% side of catalyst.
You can recoat as long as it is still liquid. Once it has gelled, the wax has already risen to the surface and you would be applying over that.
Cure time is dependent on heat and catalyst ratio, but you should probably wait 24hrs before sanding regardless. It will likely be set within 2hrs.
If you have never color matched gelcoat before, this isn't so easy. It takes very little colorant to make big changes, so it is easy to go too far. Mixing small batches is harder than large ones.
If you have any opportunity to spray a larger area from an edge to an edge, or anything that breaks up the visual, then do that rather than a spot repair. You won't notice it as much.
Mark
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