[Hogshooter]

I have seen studies done with welded samples in salt spray where metal coupons were sprayed with various anti corrosion compounds then exposed to the salt spray in a special chamber and the coupons that were sprayed with epoxy and then welded had less rust creep than any weld through primer that was tested. I usually spray the entire new part, flanges to be welded and areas that with be enclosed after welding. Before I weld I clean off the primer in the area to be welded by using a drill bit I ground flat. After welding I use a brush to let epoxy run into the welded flange then seam seal. When I have the project painted I use 3M Rustfighter "I" with a long wand to spray inside all enclosed areas (rockers, cab supports ect.) this is the closest we can get to OEM corrosion protection short of dipping the parts. Rustfighter I is a wax based solution that is self healing and runs for several days, that is the reason you have to wait until a painting is done. The way you are doing the repairs will work fine (I would still use a drill bit and clean the weld through primer out of the weld area since the primer just burns off in the plug weld anyway).