I casually asked this in my build thread, but not sure if anyone noticed the question (gets lost in the thread).

Simple question....how long can I leave Bondo unpainted?

Of course, if this was on a metal panel, then the answer is...get the panel painted as soon as possible otherwise the moisture in the air will be absorbed by the Bondo promoting rust in the adjacent metal. But in this case, the metal is not a concern since it is all completely encapsulated in fiberglass, well below the Bondo. So in this case, how long can I leave the Bondo (over the fiberglass) exposed? It is kept inside my garage, and the humidity level isn't especially high here.

The main problem is that I don't own a compressor or paint sprayer. It's a big purchase that I'm not quite ready for yet. I don't want to use rattle can paint since that will likely have a detrimental effect later when I paint properly. My friend suggested using Eastwood 2K aerosol, which is essentially similar to painting properly with a compressor...but the cost of the (single use) primer and paint would be $50+. Add shipping, exchange rate, etc, I'm suddenly up around $100+ CDN. For a single part, this seems extreme....I would almost be better off biting the bullet and getting the big compressor/spray equipment.

Can I just leave it for now and then make sure it is heated up later to ensure all moisture comes out? I have no experience in this area...any thoughts would be appreciated.

Seal it or trapped moister after paint can/will blister the paint when sun heats the panel.

is a pain but prime and paint w/ spray cans if going to drive because road salt etc get inside any small bondo holes.

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

If you are not going to drive the car, and your garage is reasonably air-tight, no paint is necessary....especially in low-humidity winter weather. I would cover it with plastic to keep oil and grease off the surface.

Just coat it with spray can paint or 2 part catalyzed primer. Bare filler soaks up water like a sponge. So does regular primer. I always laugh at people who spray can primer over rust spots to stop it. It only makes it rust faster. I usually just use any old gloss spray paint. I wouldnt advise putting plastic over it as it traps the moisture in the air underneath promoting more rust...unless your car is outside in the weather. With just a coat of glossy cheap enamel spray paint, when your ready to work on it again, it cleans off with thinner or light sanding.

Thanks everyone. I'll likely go the cheaper route since I already have lots of regular primer/paint on hand. I'll just take extra care when it's time to paint the car fully and make sure I sand it all off. Rust isn't an issue here...just making sure the Bondo doesn't sit and absorb moisture while I complete the rest of the build.