If you do your lay ups with an unwaxed laminating resin, oxygen in the air inhibits the outer surface - if your whole part is gummy, no matter how much you sand, then you might want to check your resin : MEKP ratio as that would indicate an undercatalyzed part.
An unwaxed resin system allows multiple lay ups without having to solvent wipe and sand between subsequent lay ups - great for larger projects or if you have to build a part in stages.
By the way, you can get waxed and unwaxed gelcoat (I use unwaxed).
To get the gummy outer surface to cure if it isn't undercatalyzed, wipe a coat of PVA (available from your fiberglass supplier) over it and let it dry overnight. PVA is water soluble, so it just washes off - or wet sand to keep the dust down and the PVA will wash off during sanding.
I guess you could give it a light coat of fast-drying paint like lacquer and sand the part but I've never tried it...
Regarding polyester versus vinylester versus epoxy, the stink of the ester-based resins is pretty bad; I wear an OSHA 2-stage respirator made to trap solvents, no matter what resin system I use (including epoxies) and try to dilute with added air - open a window in your shop occasionally. I also used to run 2 passive air scrubbers but never really found they worked all that great.
Each system has its pluses and minuses and keep in mind there is a big difference in resins from different manufacturers and even in the same family of resins.
Pick the appropriate resin for the job. I use iso-polyester for most of my lay ups for better fluid and environmental resistance, has less shrinkage, shows better mechanical strength and I believe a lot of the marine builders use it these days for the same reasons. I paint the inside surface of my parts because all resin systems are a little bit porous and most of my parts are on street bikes these days, so might as well make them last, right? The downside is the cost - about 25% more than a basic polyester.
I don't use vinylesters all that often due to expense, but they work great for fluild resistance, will work with carbon fiber and also offer significantly better mechanical values over a polyester system - the best ones (eg, Derakane) approach epoxy resins as regards their mechanical strengths.
Epoxies tend to be a little more difficult to use, have pretty high cost but offer great mechanical strength. I use them for the majority of my aircraft work and most repairs regardless of the part (better safe than sorry, right?). Just because you can't smell them (very much), don't think nothing comes off the lay up. You can get sensitized to epoxies by both inhalation and contact. Symptons of sensitization include difficulty breathing and puffy eyes, while contact sensitization results in dermatitis - which I'm getting over right now even though I wear good quality disposable gloves.
Epoxy sensitization got to be such a problem in the homebuilt kit plane industry, at least one manufacturer redesigned his product line so the parts could be bonded together with vinylester - no, you just can't switch willy-nilly.
IMHO, sounds like you're doing a lot of the right things, learning a lot and trying to build a few unique parts for your ride - kudos to you!
Just make certain you wear the correct personal protection regardless of the resin / part / whatever you're working with.
Regards,
Pat Cowan,
Vintage Motorcycle Fiberglass