Budnate,you effectively answered your own question. If you weld in patch's or pour in goo the patch must not be allowed oxygen or it will rust again. Any good body shop seals the back of a patch will a sealer of some sort and the outside with a good sealing primer.... anyway, thats what my buddy that paints cars advise is. On
www.Camaros.net there are alot of good tips on this particular question if you do a search.