Parsons Re-Paint used to be easily available, even from Halfords, and could be applied by brush, roller, mohair pad or of course spray. If I recall correctly the thinner for spraying was plain white spirit, certainly there was no need for the precautions necessary for two-pack. It seems to have been bough by Jenolite, and I can't find a source right now, but I think it must be similar to Tekaloid. There was also ICI Brushing Belco. These enamels had certain very obvious advantages for DIY use, and I had good success with Re-Paint many years ago. See below for Tekaloid info.
http://coachpainting...o/tekaloid.html
I will probably try Tekaloid the next time I am painting a vehicle, something that I don't particularly like doing. Nowadays I would start with etch on bare metal, or aerosol zinc on grit blasted bare metal, but last time I painted a car, that was not so commonly used. You can paint over either cellulose or acrylic (or presumably two-pack, epoxy etc) with enamel, but not the reverse. I think I will try a roller, encouraged by a number of successful instances on this forum.
I did try Re-Paint once in an airless spraygun a long time ago, and the results were very encouraging, but it turned out just as good by using a brush.