OK, I am a little lost too.
So you want to clean up the wheel arches and try and prevent any rust?
If you are capable of doing it yourself then your better off that way as it can be quite time consuming and you can be as thorough as you like with the occasional beer break in between.
Remove wheels, get yourself some Degreaser/ brush and a tar remover. Job done.
Not getting the rust part... But you are unlikely to find any rust on the underside of the wheel arches as it has been undersealed. You could always apply another coat of underseal once the area has been cleaned.
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