The Jalopy Journal
Discussion in 'The Hokey Ass Message Board' started by painkiller, Nov 27, 2010.
Is the line a crack?
I've always used Duraglass, but I've wondered about the All-Metal. I may try that on the car I'm working on. Either way, I feel that you need something between the MIG weld line and the Bondo.
Not a crack, just a very faint line that can be seen thru the paint at certain angles.
So instead of having a metal finished seam I should hammer it down and fill it with Duraglass/ All-Metal ?
I assume that the welded seam you mentioned included new steel as in a patch or replacement panel. What gage steel was used (did it match the original steel thickness?)
I'm still trying to figure out what generated the "ghost line". LoL. Is the consensus that the underlying metal or filler has cracked/opened up and moisture is wicking through causing the discoloration in the finish?
By the way, where exactly is this seam on your vehicle?
The metal is the same thickness. A new GM quarter panel on a 09 Malibu.
Antny, it is the sail panel seam. It is not discolored just a very faint line. Kind of like when you write on a stack of paper and you can see what was written on the next couple of papers.
Over any metal work I always use a product called ALL Metal as a base , then a thin coat of plastic to finish . All metal is exactly what it says and uses a epoxy hardener to cure it .
Who's welding wire are you using? The Chi-Comm. shit now on the market isn't AWS spec. and nobody knows what's in it. Too, HSLA steel in late model vehicles won't take overheating by torch or grinder. I'm guessin, since you're in the collision business that your welds are up to ICAR spec, and that's not an issue? Overgrinding an already brittle mig weld will produce this problem more often than not.
If you're in the market for more trouble slather some of that glassfibre cloth then cover your ''repair'' with some of that metal containing filler & top coat with a dark colour & sit back and watch the ghosts really haunt your work!
Not an option on the '09 Chevrolet, but I'd ditch the ''squirt gun'' welder and pick up an oxy/acet torch. I've never had a problem with gas welded/hammerwelded beads in sheetmetal no matter what colour was shot over them!
" Do not reach greedily for the Kool-Aid "
Interesting question you guys pose about the different weld types. So do you think the different shield gasses are changing the properties of the metal? Or are you just speaking from a penetration stand point? Or maybe your saying impurities are being introduced into the metal causing density problems in the metal. Or could in be pre-nuclear testing steel versus post nuclear detonation steel. That last one was just a joke.
Funny I posted a while back with the same question got like 30 different answers,no one seems to really know.I used to gas weld everything up until about 1980 and mig everything since. Iv seen lines show on some and not others.What puzzles me even more with yours showing in the cold,,the lines seem to come and go but seem to show when hot and go away when cool.
I planned on last summer doing a test panel with the different welding processes along with butt and overlaps painted black and layed out in the sun to what reacts how to try and figure this out. I will next year,,bugs the sh*t out of me not knowing for sure,,,,jim
if anyone dosen't think a MIG weld is hard, try drilling a hole in one.
MIG welds aren't inherently hard. Weld hardness depends on the material being welded, and the rod/wire used. As far back as 50+ years that I know of, and pretty common today, body sheet metal isn't always just plain mild steel.
For a lot of body related work I like TIG with silicon bronze rod. It's not as strong as steel, and doesn't tolerate flexing as well as steel. On the other hand, it us easy to work, doesn't require as much heat, and compared to steel it flows a little which can be helpful. It is more than adequate for most body welds and is commonly and successfully used for many purposes on race cars and prototypes.
If you ever tried to hammer weld you know with out question that Mig is extremely hard, even when using er70-s2. You can weld with ER70-S6 filler on the same substrate with all three process using MIG wire as the filler rod (so you have the same exact filler with the same base metal) and you will see a vast difference in the hardness of the weld MIG being hard as a rock, Tig being soft enough to hammer reasonably flat, and O/A being very soft and hammering flat fairly easily. (try it yourself if you have access to all three welders. I used to swear by TIG until I really sat down and compared it to O/A and tested it myself.)
Gas welding will leave a seam that is as close as possible to the surrounding metal in respect to hardness, thickness, and rate of expansion. All with out grinding. Tig will be pretty close, but requiring some sanding or light grinding to get the thickness the same at the seam. With Mig you will never replicate the surrounding metal no matter what you do, its just far to hard of a weld, even if you try to anneal the weld it will never get as soft as the surrounding metal.
Hahahaha ya, that's it. I was reading this thread confused about a ghost line an asked my hubby who asked me that....so I asked.
I've never seen a weld do that so I was curious....but after reading this thread, I get it now.
If it's not a full pen (penetration) weld then you can get longitudinal cracking like that. Also, or even more so, if you grind it too much. I would make sure and correctly gap it to make sure I had full penetration and then grind both sides down being careful not to take too much off.
Also, my theory on the hardness of MIG welds is because of the process itself. MIG rapidly heats up and seems to cool faster as well. The process is one in which you pull the trigger and (unless your MIG welder has a run-in feature) you had better be moving soon because the filler wire is being fed immediately. In TIG and OAW you still have to start your puddle then add your filler. To me, MIG gets hot really quick but cools quickly too which essentially tempers your steel. The faster hot metal cools, the harder and consequently more brittle it becomes. When you weld with TIG or OA, it seems to take longer to both heat up and cool down. This slower cooling process will produce a less brittle, more malleable weld.
That's my theory, anyway.
Use DURAGLAS over the weld and then a regular filler over that.
Still haven't seen an explanation of what this "line" is! How can we tell him how to cure the line, if we don't even know why it appears? What's causing the appearance of this ghost?
Flux inclusion reaction, the reactive flux core has stayed impregnated into the metal, only a trace, the cover coats reacted and it has taken this long for the oxides and hydrocarbons to create gases from the reaction to penetrate the paint surface, resulting in a crack.
This will sound weird, but I don't think its been mentioned...Magnetic field...This can happen from electrical [mig/tig] welds but gas [oxy/acet] can cause it too ..The ghost line is a color change and not a shinking issue and it can come and go from magnetic intensity change... If you can get to the backside of the ghost line place a magnet and see what happens..Or, straddle the weld with a horseshoe magnet and sprinkle iron powder and the whole weld will probably show up and the ghost line will be right at an edge of the weld....
Very interesting, when grinding MIG welds, I have had steel filings collect along the weld seam, looked as though it was magnetized. Could this affect metallic paint in some way?
BTW, started TIG welding on a regular basis recently, and I can't imagine EVER MIG welding again.
You live in Detroit? Crazy weather fluctuations cause condensation. Always seal the back of a welded patch panel. Probably the only place I like POR 15.
I had a car in DP40 left in a n unheated garage. I went to look at it one day and it had the usual East Coast hosed down wet look after a thaw. Funny thing is you could see the outline of every weld on the body through the plastic.
Ever since then I try to do all plastic over epoxy and seal the back side. I also agree that the grinding of mig welds is harder than most think. To do it properly and not too thin takes a lot of effort.
pretty much same thing happend to one of my old fenders. i didnt grind it to much(wich is what i think is this case) i thought it was that the weld was stronger than the rest of the metal and the fender flexed so i assumed it was the surounding metal cracked.
i seen something like this mentioned above.
That's great news! So there is a progress report on the '36?
Not much, fixing extra parts with TIG and practicing TIG, that's about it. Fitting and welding in the passenger quarter this week hopefully! How about your roadster?
dude, it's a faint line that's showing up in the paint. imagine drilling a hole in your fender and then welding the hole you drilled out back in. you do a perfect job of welding it and then grind all the welds down and finish everything until it's perfectly smooth. so you can still visibly see the work area but it's totally smooth. now you paint it but after some time you can literally see the work area through the paint which doesn't makes sense because you ground everything down smooth, right? so in this example you'd see a "ghost" circle appear in the paint which is coming from the metal underneath. basically something in the sheet metal has enough of a presence to faintly show through the paint. i see it all the time on bondo spots and what not.
the whole point of this thread is to figure what caused the ghost line and how to fix it. obviously there are a multitude of possible causes with just as many fixes.
hope this makes sense now.
I thought it was a color change bleeding thru as well........but I'm a piss poor welder so I'd never go there.
My .02 - Without knowing your welding knowledge/background, sounds to me like no penetration and the weld was (almost) completely ground off. Once the body flexed just a bit, it *popped* the seam just enough.
I've found lots of people don't use enough heat when welding on sheetmetal. ESPECIALLY when it comes to a MIG weld. Too scared to warp the metal maybe?
Just look at half of the "chopping the top on my xxx" threads on here and you'll see what I mean!
BTW, you can metal finish a MIG weld just as nice as a TIG or gas weld. It just takes a little more time/effort and doesn't like to get bumped around as much, so you have to be a little more careful.
The differences in weld hardness are almost entirely due to the rate at which they cool.
MIG puts the heat in fast in a super concentrated area, so the puddle forms very quickly which limits the amount of heat that creeps into the panel. The overall amount of heat in the panel is very low with MIG, so it cools the fastest, and the welds therefore are the hardest.
TIG puts the heat in more slowly than MIG, so the puddle forms more slowly, and you end up with more heat in the panel overall. More heat means that a TIG bead cools slower and thus is softer than a MIG weld.
Oxy/Act takes even longer than TIG to form the puddle, so the panel sees more heat and cools even slower yet, ergo you get the softest weld of the three.
The different amounts of heat inherent to the different welding processes can plainly be seen in the amount of discoloration you get on either side of the weld seam (the HAZ will be discolored, but the unheated material will not be).
As for the ghost line, I got nothing. Sorry man.
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