The paint I got from an automotive paint supplier is one pack polyurethane not the two pk variety. It is thinned with standard cellulose thinner, it does not require a hardener.
Now, it will probably be pointed out to me that this stuff is not "real" polyurethane & shouldn't be used on an aeroplane. However It produces a perfectly good surface finish, it takes a lot longer to dry than cellulose so keep the flies off, it does not wash off with avgas, whether that means it is fuel proof who knows.
I am not a chemist & know not a lot about this. Today is Sunday, tomorrow I shall go & ask the supplier of this paint what horrific consequences are going to befall me both short term & long term if I live to be 150. At the moment I am quite happy to squirt this stuff around my workshop at small inspection panels, aileron pushrod covers, tailplane brackets etc,without wearing any mask. If I intended to spray a complete aircraft or car I would use a particle mask & ventilate the place.
Isocyanate based systems are a different can of worms. They may well be vastly superior to anything else. Aircraft have been painted without using them for decades without falling out of the sky as a result.
Using something that is not labelled,
"Aeroplanespecialexpensivecanonlybeappliedbyexpertsatvastcos t" is not going to cause the a/c to crash, at a full respray time it will all be removed to bare metal.
Why make a song & dance about it?
Short version. Dont use isocyanate based paint.