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

[442_Mustang]

Hello all, I was referred to this site from a link regarding a problem I am having right now so I figured I would join the club
I am doing up a 72 Cutlass S and I'm having problems with the welded seams on my quarters showing up in the hot sun but when I drag it back in the garage it smooths out again. I may have a solution to the problem. It involves some rewelding plug welds along side of the seam and blasting it and using some special marine grade body filler that sands like a rock but is very very ridgid and will not flex. A guy I know does custom and restoration work and he had the same problem on a GTX he did and he redid the car over 10 years ago and it still looks perfect. I will find out more and keep you posted if anyone is interested however I am going to wait till later this summer to redo it. I've got a lot of put together to do and I'm not ready to attack those quarters again. Here's a couple pix of it as of last week. I just sanded and buffed it this saturday and sunday I drug it out to see if I missed any spots buffing it. It looks awesome while it's in my garage...