My collector box had rusted through a touch, only about the size of a 5p, so i removed the pipes and had a mate weld a small plate over the hole, then dressed it up a bit and cleaned the pipes with a wire brush and a final rub-down with fine emery paper.
Bought a can of High temp paint from B&Q, the stuff used for stoves and used all the can on the pipes, the can was good for three decent coats.
Still looking good after six months use, although i do also have a fender defender jobbie on the front mudguard, so that keeps a lot of crap off the pipes.