The rust converters work by using dilute phosphoric acid (sometimes tannin as well as/instead of?) to give a protectived iron phosphate coating - In my experience, they work well and are easy to use (wear eye protection).
The rust removers do exactly that - if it's a rough, pitted surface it will end up rough & pitted (minus rust) which can be good for paint adhesion. I used some 10 years ago on the outside of brake drums and with red oxide, undercoat, and satin paint there has been no return of rust (FLW's). Takes a lot of time/brushing and mess though (a jelly version is suitable for chassis or similar.). For small parts the fluid version is great, as you just leave the parts submerged for a day or more (which is very easy of course).
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