PH wrote:Your link is to a water based acrylic and Jackob's was to a solvent based enamel, you might need a paint expert to confirm but I'd have thought the enamel would be tougher and better suited to outdoor use. Preparation - very light sanding, try and smooth the edges of the existing paint, several thin coats rather than a thick one, a thin primer if you like though I'd probably not bother. You haven't really got much to lose, if six months down the line it's looking like it isn't going to stay on, you can always rub it off and start again.
Only just noticed this post of yours. Yes, you are right. The info for the acrylic does say it can be used on metal.
Hobbycraft were out of the enamel Brunswick Green when I popped into one of their stores, blank space where it should have been (and in fact their web site now doesn't show enamel at all) - I picked up the acrylic but will get some of the Brunswick Green enamel as well to compare.
Plenty of places to practice on the bike - modern paint jobs are so delicate that even locking bikes with them through the rear triangle is pretty much certain to produce scrapes.
Thanks PH for pointing this out and all for your advice and reassurance.