Degrease then rub down with emery paper to create a clean and smooth substrate ready for painting. Primer depends on the topcoat. Some paints like Hammerite don't require a primer but they aren't cheap. Hammerite smooth coat may be an option. I have rubbed down and reprimed my (steel) rads at home and top coated with gloss. Google hammerite as a starting point.
Nah not really....its more about the product you use...is it suitable for the application.
Many of the products on the market for this task will be ok. Just check if it requires a primer...many do. If you can post in here youll easy find a product for your rads on google..toolstation and screwfix and wickes will have them.