I am not an experienced painter by any means.  My general competence level is that of painting old farm equipment with a brush using whatever enamel I could find.  However, I want to start to learn how to paint for real, and am planning to put some time into cleaning and repainting our D17IV as a summer project.  Obviously not to a show quality, but to a decent level.  I plan to use acrylic enamel.  Over the winter I've read up on painting and even have a few basic books.  The isocyanates thing in a hardener makes me worry.  I know that for the two stage urethanes and such, full body protection with supplied air with good exhaust is necessary.  However, for regular enamel, painted outside or with good ventilation, it seems a good respirator mask with the proper carbon filter material, plus the normal gloves, is sufficient.  However, I have read conflicting opinions on using a hardener.  It is recommended for improved gloss and durability.  However, some say that even using a few ounces changes everything, while others say a good quality respirator mask is sufficient for the small amount of hardener present in the enamel.  I prefer not to ruin my lungs for the sake of a paint job.  But a full air-supplied suit seems like overkill.  However, if that is really what is needed, then I will either re-think starting my new career in painting old tractors, or give the work to a professional - or leave the hardener out and just use regular enamel. 

I know this is probably equivalent to an oil-thread, but there is so much collective wisdom here, I trust the opinions.  So in reality, how dangerous is it to add a hardener?

You can probably tell I'm not the "doom & gloom" painter at all. I have a downdraft paint spray room with heated make up air, etc. to satisfy the insurance requirements of a repair shop operation, but don't even bother with anything but the lights and exhaust fan to rid the the airborne overspray. Clean clothes and a fresh blow off with an air gun just before starting to spray is about the only precautions I take. I don't even use rubber gloves because I am still a man. The key to a good job is preparation with the materials selected, cleanliness, and technique. All require practice; at times, a lot of practice. What comes natural to some doesn't to others.

It's been over 30 years since I've used acrylic enamel with "hardener" dumped in. Used to spray a lot of DuPont "Centari" enamel with 793S hardener reduced with 3602 acrylic lacquer thinner, allowed to set after spraying a couple of hours, and shove the vehicle out the door. You won't read that in any text but experienced painters from the era when those products were popular, (still available but not widely used due to VOC content) will tell you that was the ticket for getting a job out the door quickly. I cite DuPont, but any acrylic enamel, and fast activator/hardener would work.

An activator, or hardener in acrylic enamel with outlast in shine the same paint minus this ingredient. Most will tell you the paint with flow, or level better with the additive but that depends upon technique, (IMO). It will also cure much faster. I never intermixed generic products in any manufacturer's paint products as couldn't afford the comebacks from "experimental" paint failures. Spray guns; there are a plethora of them out there. I have spray guns on the high end, and the low end. Find something that is comfortable in the hand, and sprays a 12" - 14" fan pattern at 12" from the surface evenly, and fully, with about 40 psi at the gun with the trigger pulled back. Set your gun to comfort level for speed traveled while spraying and use a 50% overlap in your application technique. These are basic guidance practices as everyone is different. Remember this when setting your spray gun: Set the fan , fill the fan. What that means is spray some paint against a piece of masking paper and set the fan spray pattern for the desired result. Then adjust fluid flow to fill that pattern. Pull the trigger back, and release, evaluate what you just sprayed. When wet, evenly filled, and not running down the paper, start with movement to ensure even coverage and no blotch. Takes practice, practice, and then a bit more practice to get comfortable. Even application is the key to a good looking finished product. Has to spray wet enough to flow and level, but not so wet it runs from the surface. Horizontal surfaces don't run right? Lay paint on really thick and it fails and/or dies back in gloss from solvent entrapment. Same application thickness no matter where you apply. 2.5, to 3.0 mils is about perfect. 

Acrylic enamel is very subject to hazing from humidity as it dries so try to paint when the humidity is relatively low. Good airflow over the vehicle helps but painting when it's raining or 90% humidity can have detrimental effects. Enamels are also subject to "fish eye" from the surface not being very clean and free of oils/grease before spraying paint. Clean, lint free wiping cloths and fast evaporating solvent preclude this. 

When I do paint, I use AGCO (Valspar) acrylic enamel paint with Valspar Renew hardener (I get the paint from a local AGCO dealer and the hardener from a John Deere dealer). 

The AGCO paint is reasonably priced, sprays well, shines nice and doesn't fade. 

I do most of my painting in the winter and I have a small exhaust fan that I put in a window (it has to be small so, I don't lose much temperature). I use a 3M full face respirator that has replaceable cartridges. I have had no problems.