After electrolysis, how do I prevent flash rust?

[In The Ten Ring]

Well electrolysis seems to work for de-rusting, although I need a lot of time it seems. Once I have cleaned up a fender (which is coming next week) what do I do to prevent flash rusting?

I probably need to mention that I cannot paint it, unless with rattle can. A body shop will handle the matching the fender up to the rest of my truck.