I am not trying to sound like a naysayer, but every few months someone tries to go papier mache and they are excited at first and then toss it because it really is not strong enough to be useful. The big thing holding it back is you still need to smooth the outside, and that's too much physical stress on the papier mache. Most people either buckle and switch back to fiberglass, or go the aqua resin route instead.
As for just sticking with resin, the best two things you can do are: rest it, and then seal it. The resin "mostly" cures in the first day, but it can take up to a week to really waft out the lingering resin fumes, it's incredibly noxious stuff while it is wet, so much that it lingers. After a week, it should be reduced enough, and you can get a heavy duty paint and hose down the inside. This will coat it with a layer of paint that keeps any more from wafting out. What that means though, is you should make sure you finish any resin/bondo/rondo work you want to do to the inside of the helmet. I actually suggest doing a thin layer of rondo over any fiberglass, as bondo tends to not stink as much as resin, so it can help further reduce the fumes. But definitely air out that for a bit and still spray paint the inside.