What is the best way to clean the inside of fenders thave have 60 plus years of build up and very minor surface rust?

I just talked to a guy (owns a resoration shop) that said clean it with gasoline/kerosine and scrub with wire brush. After that use POR15 and leave it like that since there is not reason to take it all the way to the metal if it is going to be a daily/weekend driver. Thoughts?

Chris