Thanks for the advice, the last panel i did was a warping nightmare. I do not want to do that again. I ended up getting a lead/tin kit and filled most of the warpage from my buttweld. Then did a very thin coat of filler. Probobly not the perfect way but it was a practice car anyway and had very thin sheetmetal. I only had a few blowthru's and I bought a nice copper backing kit with a handle and interchangeable shaped copper ends and finished it up.
I think ill do it the way you told me to do it and use some weld thru primer on all the "to be welded" areas and between the flanges. Then when its done ill hit it with some epoxy primer, then finish the backsides with paint just as I would the front but also apply an undercoating or herculiner of some sort.