The fastest way is to soak the whole thing into paint remover. It will dissolve everything depending on how long you leave it in there.

If you want the paint to stick perfectly, priming it would be best. However, we've painted rads with no primer and it will stick just fine. It depends on what paint you use.

Off the shelf - Bosco

No primer needed. From the picture your paint job looks fine. Some paint are very heavy and dry very slowly. Which in turns become runny very quickly if you overspray. Those Bosco paint are really good, more impressive than Krylon. The best is still going with HVLP

Yes, HVLP.

Bosco can be found at certain hardware shop, don't remember where was the last shop found. This is by far one of the best fast drying low pressure off the shelf paint we have used. You can use anchor/ ABC, or whatever brand. Anchor is actually not bad.