i deal with rust a lot and there are a few ways to handle it yourself...
use a rust remover which is probably a phosphoric acid in a liquid or gel form that you paint on and let it dissolve the rust then wash it off really well and immediately apply a good primer which is high in zinc...primer used in marine applications usually have a very high zinc content
or, you can use a rust converter. wire brush all the heavy rust and hit with a light sanding then apply the converter (brush on liquid) and it reacts with the rust and neutralizes the rust and has a bit of a protective seal when it dries. the converted rust needs paint to protect it and the dried converter acts as a primer, but i would use a zinc primer over it and then paint it afterward for best protection
use a coating like POR15 which kills rust and dries to a very durable finish. you can get it in black so it would be perfect for a vehicle frame
not sure what you have in the UK, but here are samples of what we have in the US
i have all three of these that i use depending on the application. the POR15 is probably your best bet for a vehicle frame
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