General Technical Questions and Answers last answered over 1 year ago.
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rub down with wire brush then coat with curerust as instructed on the tin then primer or red oxide, followed by hammerite or similar if not worried by colour.
Personally, I would just not trust a garage to get ride of rust. The only way rust can be treated is to remove it 100%. I know that there are treatments advertising that they neatralise rust - but they dont.
There is one chemical that will eat away rust and leave the clean metal and that does work but you need to imerse the part in the chemical which is not very practical for your bus.
I spent most of last summer slowly working my way around my van with a dremmil, grinding away the red stuff. I know how many hours I spent doing this I would hate to think what the bill would be from a garage.
So get yourself a dremmil and loads of the little grinding disc's and set to it. The only person you can trust to do the job right is yourself.
Once you have removed the rust and suitably primered those area's take it to a garage and have them finish the bodywork.
i agree with splendiff, a garage will love to take your money, lots of your money.... bet you dont get a guarantee, you can get that rust remover in a gel, so it dissolves it.