rust inhibitor?

[Suicide-D]

I'm in the process of switching my doors out... the ones on my truck are junk... with holes for speakers and cracked and dented... I have a pair of fairly solid doors off a 72. My plan is to fiberglass the bottom of the door where it is rusted. I noticed yesterday when I was stripping the worst one of the two... that the inside of the door as alot of surface rust.

Is there something I can spray in there to eliminate this rust? I then planned on spraying a rubberized undercoat or bedliner into the door...

If I have to go to alot of trouble and expense to do this... I'll just buy new door shells... I really think I can salvage these though.

Thanks