weld through primer

I'm sure there is a thread about this that i couldn't find (i did look really). I'm helping my cousin with his 70 gtx we have sand blasted and cleaned the frame and trunk pans and used 3m weld thru primer on the tops of the frame and the bottom of the pans. the problem is when we started to spot the pans in the welds splatter and burn thru. everything is nice and clean and close i'm using a millermatic 135 with gas. I have done some of the same things to my scamp with no where near as clean/good metal. does anyone have any ideas what would be our problem and maybe have a solution