Do you mean oxidisation of the barrel like this?
If so then ideally you should take the engine out BUT you can make a decent job in frame but it's a bit fiddly, though less fiddly than taking the engine out. Take the tank off, scrub as much loose stuff off the fins as you can with a stiff brush and use a vacuum cleaner to suck away the dust. Mask EVERYTHING off, frame, engine cases, carb, cables exhausts anything that you don't want overspray on. Next I used Sperex VHT silver exhaust and manifold paint, short light squirts working into the fins, give it a couple of coats - don't let it run. Let it dry and remove the masking, run the engine - it'll stink a bit until it cures - and that should be it. I did mine 3-4 years ago and it still looks good, it's not the ideal solution , but if you take your time you can get a pretty good finish. If you are really careful you can get the inner crankcases too.
