The best thing I have used so far is Fluid Film. You apply it once a year to the inside and out of the frame. It takes minimal prep, just remove loose paint and rust. You can buy it from the distributor Kellsportproducts.com. An undercoat package is about 80 bucks and includes 1 gallon, 1- 11.5oz spray can, a spray gun with 2- 1qt bottles, a small brush and a 2 ft spray hose for the can. Shipping is free.
Scott