Classic/Antique Car Repair: Phosphoric acid, rust repair, zinc chromate primer, hot air gun

Question
Sir,
Please advise me on the correct use of phosphoric acid as a rust remover? I have read on the internet that it is effective in removing rust, and leaves a layer of phosphate on the metal surface, which resists rust. However, I find that rust appears on the metal immediately after I wash the acid off. Am I doing something wrong?
Also, would you advise using a zinc chromate primer on surfaces where it is impossible to grind off all the rust? Do you support the use of rust convertors?
Any advice would be greatly appreciated.
thank you
Alan
Karoo, South Africa

Answer
Hello Alan, you asked:

Please advise me on the correct use of phosphoric acid as a rust remover? I have read on the internet that it is effective in removing rust, and leaves a layer of phosphate on the metal surface, which resists rust. However, I find that rust appears on the metal immediately after I wash the acid off. Am I doing something wrong?

No you are not.

If you apply it to clean or new steel, the same will happen. It does seem strange I know, but it does show you that it has done it;s job. The 'rust' you see is the phosphate.

If you use hot water to rinse it off, the metal temperature will increase and help to dry the metal, prior to priming etc.. A hot air gun will also do same.

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Also, would you advise using a zinc chromate primer on surfaces where it is impossible to grind off all the rust?

Zinc chromate primer is great for coating new metal prior to spotwelding. I do not think it is suitable to treat rust. Use the first option, this will neutralize the rust action, but don't grind too hard trying to remove all rust, as you will weaken the metal. Let the acid do the work and then prime, paint or underseal, etc..    ;-)

Do you support the use of rust convertors?
Yes, definately!

Hope this helps?
Best wishes,
Brian
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