A product I have had excellent results with is OSPHO. About $25/ gal and it goes a long way compared to POR 15. You should coat it after, either paint or oil. I try to use 90 weight cause it works longer than oil even after thinning with diesel (but it'll stink for a week). A quart of Ospho will do it with 1/2 left over. Remember to insert a tight fitting tube inside the frame at least a foot long before you weld, maybe even a few plug welds at each end of the splice fittings. Hope this helps you.