If your welding the new metal to any rusted spots it will pop like hell . Keep cutting away until you get good metal and then use the ZINC primer ( be careful not to breath that gas when welding).
I`ve used all brands of seam sealer
and found 3M brand to be really "stringy" , you leave off the seam and it`s like a thousand little hairs follow off the job , really annoying . I found PRO-FORM to be the best by far . Hope this helps .
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