Thanks, Alan - I do spray with an OV mask, which seems to be enough for something like the crosslinker in target, but not so much for isocyanates from what I've learned in the last day or two.
i don't need the finish to level or lay out perfectly, so the system 3 finish you mention may be fine. It's typical to lay on top coats for instruments, and then wetsand/buff them back to a more glassy finish. Something that will look similar to automotive clear coat and be tough over automotive type paint (like the original fender guitars, though for obvious reasons, those aged) is my goal. wiping or brushing is OK if that's the appropriate way to deliver the finish.
Outdoor application is tolerable if "open bug time" isn't too long.