Ghost line on a welded seam

Mig welded seam on sheetmetal (butt-weld) . Ground pretty smooth, skim coat of evercoat putty, urethane primer, Dupont chromabase and urethane clear.

2 months later I can see a ghost line where it was welded.

What did I do wrong?

 

Was it a full pin butt weld or no?

 

Hate to say it,but sounds like you ground it too thin.

 

what brand primer, it doesnt sound like you used a sealer so the base could be biting into the bodywork underneath.

 

not a good enough weld ( has to welded solid, even tack welds every 1/2 inch will split) or like metal man said ground to much of the weld off.

 

Seam was welded solid . Must have just ground it down too much.

What is a full pin butt weld?

 

Full penetration butt weld I'd say ??

 

Maybe the weld expands and contracts at a different rate than the surrounding sheetmetal when in the sun or cold.

 

MIG on sheetmetal.

 

x2...

 

It is really best to give the materials time to dry . If you do a job like this in a couple days primers will shrink up . Great to let them set a week before you do any top coating .

A good sealer is about the best thing to hold things back but then let the primers dry for a few days .

 

like bluestang67 said mud and primer both shrink i always take my primered car outside every day for a good week and let it bake in the sun then wet sand let it sit some more then primer again and repeat, paint bake booths some times do the same thing leave ghost line if done to fast from mud primer sealer base clear

 

I have had that problem before. I thought it was the filler shrinking so I started using dynaglass fiberglass filler first then regular filler to finish it. Cured it for me. Hope this helps.

 

I'm a retired body man and this is unfortunately a common problem with a million different legitimate answers...I always (if possible) put a piece of sheet metal behind my seams so I can penetrate fully to the other side. In most cases no one ever sees the backside. Use quality body fillers and don't rush the product. Always use manufacturers (between coat) times for body materials as well as paint products. If in doubt,a little extra time between steps costs nothing.

 

All good answers, and it could also be that the MIG seam is more rigid than the surrounding sheet metal, so it doesn't flex the same, or heat and contract the same.
Problem is especially prominent on dark colors!

 

This is what I was thinking right away,... (Guess how I know ? )

 

That's not how steel works....

 

I'd like to seesome lab test on MIG vs. Gas or TIG on sheetmetal. Butt weld two pieces, grind smooth with parent metal and test for penatration and strength. My guess is that MIG is the weakest of the three.

 

if you want a mig weld to be strong, don't grind it. Pretty simple.

 

MIG seems to be the hardest of all three,... I have GAS, TIG & MIG welded sheet metal,... In my experience TIG welding sheet metal (It seems to me it has really good penetration, and easier to work down), is the easiest to work with, than GAS welding and finally MIG.... But I am in no way a expert.

 

As a body shop owner I see shrinkage of the fillers and paint on all of my toys. We don't rush them, plenty of cure time, best products ect. 5 or 6 month down the road you will see distortions in the surface.
I never colorsand and buff the finish coat until I feel that the shrinking process in complete. A year is a fair time to wait. At that time, a newly sanded and buffed wax job will renew your prized gem.

 

we run in to this sometimes when powder coating car body's.
it's a VERY thin line when grinding the proud of a weld down to meet the surface.
we have also found that both sides of the panels need to be ground, dollied, wheeled, etc so the metal is a consistent thickness front and back

 

"All Metal" filler from USC.

I use it (same process as above) on shaved door handles, patch panels and sections.

Dynaglass works also, but with dark colors on a hot day you can still (barely) see the weld line.

 

Damn , I did both rear 1/4's on my car and epoxy prime / build prime / BC-CC .Now I'M worried! It took me a while so I hope everything shrunk. I did colo rsand it right away though.

 

IMHO, it's definitely the different hardnesses between the parent and the weld............mig welded? Mig is very hard, and very hard to eliminate this problem........

 

chopolds has hit the nail square on the head. Even if you do every thing by the book but when you do your welds you used a cheap high carbon wire / rod with your mig or tig on thin sheet metal these is a very good chance you will get those sickining ghosts especially out in the direct sun on a hot day.

 

Not in the hot sun. It is right around the freezing mark here in MI.

Thanks for the replies.

 

...... agreed, could be shrinkage of filler/surfacer.

 

Is it possible to post a pic of the ghost line? I'm not sure I get what you mean by that.

 

Not the filler. There is just a thin coat of putty.
What I am seeing is a staight line. Not sand scratches and a bondo edge.

Antny, I dont think it would show in a pic. Line is very faint and on a silver car.