As above Mate, the 3m masks will help with particles in the air but airfed mask is the only way to protect from isocyanites.

It has a cumulative effect though, if you painted a bumper with good ventilation and half decent mask you'll be fine, paint a few bodies a week and you'll be done In no time.

A mask with air from outside will work fine too, so long s it has a valve so you don't breath out back up the tube the fresh air comes through cause it'll cause dead air in the pipe and after a few breaths you'll have used the oxygen and will probably go dizzy. Fresh in through tube, out through sides of the mask or summat.

You can't use air from compressor either, unless its properly filtered for breathing. The compressor concentrates anything it breathes in, anything harmful in the air will go in, including oil vapour inside the cylinders etc too.