metal 2 metal filler?

where i shaved my tail light (but didnt finish bodywork, maybe just used 3 really skim coats of evercoat everglass) on the edges of the filler that are close to my tailgate, theres maybe 1cm of a rust line at the top and bottom and a small line towards the side by the bed (the vertical area) and my friend told me it may not be because of my welds, but could be that i didnt use a good enough sealer over it all and inside the filler. he recommended sandblasting it all and using metal to metal filler. how does this stuff work?