never tryed the rad paint but all i ever use in my house is water based gloss its been painted for 5 years now and had to paint the door again as theres a few knocks on it
half way through painting you could not even see the differance between the old and new once its dry
the only problem with the water based stuff it needs more coats
For Gloss I only buy the dear stuff from local trade paint place as had it flake off and go yellow once so ended up doing it twice, I shoudl really ask about rad paint too
I have used hammerite from a tin and plasticote aerosol.
The plastcote has fairly poor coverage and has gone yellow in about 9 months...avoid.
The hammerite is hard to get a good finish and is also now starting to yellow a bit after about 2 years...though it is tough.
Any oil based paint will yellow unless it gets a lot of direct sunlight, I'll be trying something different next time though I favour just buying a new radiator over the hassle of prep and paint now.
I'd be willing to give zinnser all coat a try, zinnser products are always fantastic and I was very impressed with the satin black all coat I recently painted my CI downpipes with
I used s/h cast iron rads from a school, the plumber piped them all up to test them and they were then sent off for blasting and painting. They just had the one thin coat of high temp metal paint sprayed on them, the difference in output after they had the paint stripped off was very noticeable.
Still good thirty years later and banging out the heat, I reckon the less paint on them the better . Not bad for rads that are over fifty years old