qtr panel seam spots in paint

[fatelvis]

I have a 69 that had qtrs welded some time in the 80's very nice shape. I ground out all of the old mud and laquer primer to inspect the welds. Reapplying fresh filler at a minimum, over bare steel, the car was subsequently painted black with a very nice color sand/ polish job. Herein lies the problem, at first, I rolled it out in the sun to clean the garage and noticed a faint tell tale line where the seam was welded. Ticked off , I rolled it back in and 1/2 hour later I reinspected the spot only to find it had vanished. Now I will admit I am Picky, and yes I know black shows all, what I need to know is what do I need to do to alleviate this problem?
1 Year later, in the dead of a Kansas winter, I can see some of the seam, on both sides. The integrity of the filler is OK, no lifting or staining. Do I need to grind it back out and put a thicker coat of mud?? Use all metal?, will Lead take care of this ?
This car needs to be perfect, what should I do?
Will Gluing the Qtrs eliminate this conscern?

[watahyahknow]

might be the temperature difference that stretches and shrinks the metal ..... the filler doesnt , black holds heat verry good you could try lead filler next time maibe that works better

[beep4beep]

one queston? was the quarter welded in solid or just spot welded around the perimeter? if just spot welded grind out and weld solid. then do the body work over. best of luck.

[Alchemist]

Plastic fillers will expand and contract - and expand dramatically so due to heat absorption (because it's plastic) when painted very dark colors like black, blue black etc., Using metallic like filler can prevent this.

Block sanding is also critical when painting cars very dark colors and can really show minor imperfections that were missed while blocksanding.

Primers are the foundation of a paint job. If lacquer primers and paints were used - you will definitely get shrinkage.

You didn't mention what type of primer/paint was used.

[Doright]

Pictures please