Has anyone had any success painting 15MM copper pipe when it's used to transport collant water a round the engine bay?

why anything fancy? it's only gonna be about 100 degrees tops, unless its next to the manifold.

celly primer, then aerosol black for mine, assuming you mean on the outside
tom

why paint it polish it,

Powder coated. Cheaper than painting.

What about useing chrome pipe and fittings, you get them in B&Q

Just what I was going to suggest. Used this before on cars. It's a chromed copper pipe. Not much more than just plain copper.

I used grey primer.