I get the fit and radius as good as I can before welding, spend a bit of extra time smoothing the welds in the radius, add some welds on the inside where possible, then start the filler process with Evercoat filler with the short strand fiberglass... As Jay mentioned, fiberglass and steel move different with the heat and it's just a matter of time till the fiberglass to metal seam shows up... Good fitment and thin coats of filler is a much better way to do it!