with the epoxys mentioned you will want to weigh the part a and b on a gram scale before adding any fillers. The epoxy sold in the 2 part siringe or hobby shops 15 or 30 min stuff can be mixed eyeball measure just dont try to make it set faster by adding more hardener. Chopped glass or milled. Carbon fibers are a good thickener in a pinch some of those other fillers like cabosil you have to buy a lot more that you need. Another good thickening is microballons they are either white or rust colored sold in hobby stores also. Should be able to gelcoat over epoxy if its sanded and cleaned with alcohol.