Duster fender help

Just got some good used fenders for my 76 duster from a fellow FABO member. they look pretty good but have a little bit of surface rust on the inside. So what is a good way to clean them up ? Looking at the Eastwood rust encapsulator stuff and it sounds good. Anyone used this ?
Also notice that the antenna hole on my good fenders is different than original. What year did that change ?