When it is apart it needs to be blasted to Grey metal (no dark pits that is rust) then 2 heavy coats of epoxy before you assemble it. Most of the professionals on the SPI forum just use SPI epoxy for the “weld through primer”. Grind a drill flat to use as a scraper to clean out then epoxy in the spot weld hole before welding. The epoxy can take a lot of heat. When done try to brush epoxy from the inside and let it ooze into the welded joint. About the best you can do without a big tank of epoxy and dip it 1/3 of the way up like the factory did.
