Over the years iv done a some jobs were I sectioned a hood or quarter panel ect,I fully welded the seam,ground it smooth,skimmed it with Mar glass then Bondo, Catalized primer,blocked and painted, and about a month or so later depending on outside temperature or time of day I can see the line of the weld seam,and it will come and go. Iv noticed it only on the regular drivers that are out all the time not the ones kept garaged.. Any ideas on that? Does it have to do with expanstion rates of the products? thanks jim
I think expansion and contraction of dissimilar materials due to changes in temperature might be the culprit. Also (not implying anything here, just a thought) the thickness of the combination of materials on top of the sheet metal as well as the cure time of each component can have a big impact on these kinds of problems. Rich
By no means am I an expert, but I read everything I can find on the subject in anticipation of my next project (HAMB friendly), and I've come to the conclusion that there should be an etching primer directly on the metal. Also; there should be no reason to use the fiberglass based product, as the metal should be able to be worked to with-in 1/8" of the final shape. Any comments will be greatly appreciated. UNCLEE!!!
I think the problem is with the Mar-Glass. Resin based fillers don't have a great relationship with temperature and can show ghost lines like you are describing. Being that the seam is fully welded, there's really no reason to use a resin based filler. This is one of the downsides to it.
Yep, it's referred to as thermo dynamics in the text book we use at school. The type of weld joint can cause it, or the different expansion rates of the materials. Over lapped body panels are a common cause. Even a MIG butt joint can show if you don't get it hot enough to fuse both panels AND the skip weld beads. The tiny series of tacks can be a problem. People tend to worry too much about heat and warping and forget about fusion. That's why I mention learning to hammer weld with gas every chance I get. Just preaching the gospel of traditional hot rods and customs! MIG's suck for sheet metal!
Hmmmmmmmm. I cannot begin to count the sheer number of sheet metal parts I have mig welded on in my 28 yrs in the business and I have never had a seam that was welded expand and contract showing a line through the paint. The mar-glass is the culprit.
Yeah, I would leave off the Marglass and just seam seal the back side of the seam to keep moisture out.
TinBender, Public Enemy & Cleatus: Great info. For decades I've heard diferent theories about this (especially when doing thinner metal like, say, a '70 'Cuda). One guy was still tinning & doing lead-fill back in the mid-'70s. He finally gave it up because he said the steel was getting too thin to prevent warpage. Thoughts?
SPECIAL SECRET FORMULA FOR TROUBLE-FREE REPAIR #1 All metal correctly ground, no rust, paint or old filler on repair area. All welding is to be butt welded, if at all possible and either fusion welded or little filler rod used in weld. All damage is to be bumped into shape prior to any welding being done. #2 If you must use filler, be sure to only use the amount necessary to fill damage you can't get behind to straighten or will require more than an hour to metalfinish. The use of "Tiger Hair", "Kitty Hair","Mar Glas"etc. is not permitted! These products all contain polyester resin and do not work with other chemicals placed on top of them, yep, they will "witness" if given the chance. #3 A coat of quality epoxy primer followed by surfacer, if necessary, and a coat of epoxy/sealer over the surfacer will provide a good trouble free surface to paint over. Filler, primer, surfacer,or sealer over older/different kinds of paint finish or old repairs, all spell trouble for a paint job long term! Swankey Devils C.C. " A legend in his own basement"
Yo, Pimpin'Paint, sorry I posted my little ocmment a second before your VERY thorough advice! Great thread, 'cause I've had this problem!
what pimp dady said was , it's all about corrosion, clean it up right, and coat it before you ever sling any filler...
Me too, It can be done, and done well. I did for my first 25 years. I have however repaired many MIG welded panels that failed due to too lack of fusion because someone was afraid to get it hot. For the last 10 years I've been hammer welding with gas. Once I went to gas the difference in the workability of the metal alone was worth the extra time. It's far superior to mig for sheet metal. I've never met anyone who can gas weld sheet metal that would use a MIG to seem weld panels in restoration. It was the very best thing I've learned, and a huge improvement in my work. Weld any way you choose. I'm going to keep passing the torch every chance I get. If one or two people decide to learn to gas weld then I've done a good thing.
The only thing I would say about your comments is that #2 is a bit contradictory in the fact that you single out the glass products because they contain polyester resins but in reality almost all conventional body filler are polyester resin based. Other than that great post.
mar glass is pointless to add to a steel seam, it would be much better to just use bondo there. mar glass is mostly used for plastics and fiberglass
did you butt weld it or flange/lap weld it in? if its not butt welded the metal is twice as thick and will expand/contract at different rates compared to a butt weld..... and what everyone else said.....the glass will also cause it
Hmm, i was always taught to toss a little bit of glass in there. Reason being, as i come from the rust belt, MIG is porous, so a little bit to hold back to moisture from comming thru and absorbing into the poly filler is why, but idk, million ways to skin a cat. Now that im az, the last thing ill do is put glass on it.
Hey, Ah, but it's the amount of that polyester resin in the glass cloth products that causes the problems. The polyester/talc/glass based plastic fillers have much more "give" in their makeup and thus don't present the problems the "Kitty/Tiger MARglass" products do. These products are really for use on fibreglass boat and laminate products like Corvette repairs. I understand from my buddys who live in the land of the frozen winter that these products have been used for years to "effect" repairs on rust outs to get vehicles through annual inspections. I jus' never thought anyone would consider this method as a serious, lasting repair! I nearly shit myself when earlier this year I cut into a '53 Cadillac Drop-top only to find the entire top well's lower 25% made of glass and cloth. The owner payed over 45K for this vehicle! When I asked the owner if he got a kiss with the deal ;he was not amused. Swankey Devils C.C. " Spending a nation into generational debt is not an act of compassion"
I say every one should use Marglass, then we'll have a pool to guess when the first cracks will appear. HAMBER with the closest guess will get a case of Marglass to fuck up thier own car.
I mostly agree, the problem is probably with the Marglass, too strong (rigid). But another possible reason...not knowing your skill in welding, could be the weld seam. I've seen seams that were a bit "sloppy" witness up, too! That is, they weren't ground as smooth as they should have been, or have excesses of "boogers" on the backsides, that make the seam too stiff for a flexible area like the sidee of a hood. Same as having a flanged weld, almost.
The Jalopy Journal
