I've lost the URL for this - it's from the CTC website - but it's superb. It's for really rusty frames but still useful in general, if a little OTT:
by MikewsMITH2 » Sat Jul 25, 2009 9:25 am
I'm afraid paint won't stop rust coming through, to prevent this the rust needs to be neutralised first. This is what you need to do:
1. Strip back to bare metal using Nitromors paint stripper.
2. Sand down until all surface rust is removed.
3. There will still be rust in the pits - sometimes so small you can't see it. To neutralise this you need to use phosphoric acid. This is available in gel form from Halfords and the like as "Hammerite rust remover". This is a very weak form and will work but you may need several 2-3 hour application cycles before all the rust turns to a blueish colour. The best stuff to use is "Jenolite" from jenolite.com or "Milkscale remover" from an agricultural merchant. This is stronger and cheap.
When the rust is neutralised use "Hammerite Number 1 rust beater". This is a zinc phosphate rust proof primer.
4. Then prime and paint as normal.
This process has been used in classic car restoration trade for many years and done properly this will definitely kill the rust. I have done two rusty 1960's frames (one very rusty) and used one all last winter with no recurrence of the rust. If there is much rust on the frame it's probably best to get it media blasted, but still use the jenolite/milkscale remover as the blasting process will not remove all the rust in any pitting there may be.
Another tip is to use plenty of paint to make sure moisture cannot get through. A 2 pot epoxy paint is best as this is totally waterproof whereas convential enamles aren't. I also clearcoat the the whole frame after the transfers have been fitted. Plasticote is the correct stuff for this as it won't damage the transfers. I also wax mine with car wax after the paint has hardened. The only problem with professional re-enamelling is to make sure they are using the correct process of blasting/dipping to remove and neutralise any rust. Obviously if your frame isn't rusty then stripping, priming and painting is sufficient.