Would gelcoat fill in minor imperfections? Is gelcoat necessary for automobile work?

I added some mat/resin today. If the weather permits I will try block sanding tomorrow

No, gelcoats are designed to set and cure in an "airless" environment in the mold cavity. To make them work in normal air you have to add something like Fiber Glast's special additive. You are just adding extra labor and costs for little added benefit.

I know this is a old thread but as already said this is gonna flex when bolted in you should working it with it bolted in and work more with the glass than fillers, at this stage of game use fillers for last lill bit to get level for the lexan just my .02