Weld seams on quarter panels visible in hot sun, but not garage--how to remedy?

[68fusion]

Yes I took a lot of time to not warp the panel. The filler isn't very thick. I may have a buddy swing by with a TIG welder and will cut and reweld it

The thin filler may be causing it, not enough build to mask the expansion of the metal and non movement of the weld. If you spread filler on that area and had patches of bare metal showing in that "seam" area then it wasn't enough filler. You can put whatever kind on you like... kitty hair, duraglass, or carbo whatever but if the build is not enough then you'll have the same problem again. Metal and welds do some strage stuff in the sun.