Rustbusters say that a mask is not needed to use their 121 Epoxy mastic. So is this not a 2pk paint? I wanted to use it to paint non visible areas of my Suzuki jeep and also the metal bumper and rock rails, wishbones etc.
Two pack means 2k ('komponent' in German), indicating that there is a base product and an activator. It doesn't necessarily mean the product is harmful. If it doesn't say it contains isocyanates and says you don't need a mask then you should be OK.
To be honest though, for the sake of a few quid I would buy a decent mask. I refurbished my alloys recently with spray cans. I couldn't find my £15 Gerson mask so used a small paper mask. No mention of a mask required on the can but the fumes were very strong and unpleasant and I could feel the effects on my chest for the rest of the day. If you are doing a big area a mask is definitely worthwhile.
I did my wheel arches in Gravitex. Don't think a mask was strictly required but I was glad I used one. You can keep going for longer and get the job done more quickly.
If you are spraying then wear a mask, brushing then no real need if you have good ventilation. I spray 121 with a 0.8mm nozzle touch up gun and there is very little overspray but I always wear a mask.
121 does not spray very well due to it being very thick, the data sheet says use an airless spray.
For non visible areas use a brush, no need for a mask.
I have sprayed it with through a 0.8mm tip and there was little overspray so if you spray outside you can get away with a simple face good fitting face mask.
If I had to spray indoors I would use my air fed only.
I did this at the edge of my garage with a simple mask.
Make sure you wear gloves at its a pain to get off your hands. I use a 3m full face mask with doors and windows open with and extract fan with ducting close too the parts I am spraying ..