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Thread: Pitfalls of flanged weld seams

  1. #11
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    I've always wondered if that is why gas welding was a preferred method that has used by some builders since the early days. The welding temp is much lower, and the weld more ductile.

  2. #12
    Registered Member NickP's Avatar
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    Quote Originally Posted by shine View Post
    in the early 80's i owned a van shop. we painted 10 per day . at the end of the year all that was left was 2 tone chevy duallys . there was a 3/4 inch tape strip . we removed it, da sanded with 80 grit primed and painted . outside the 3/4 stripe would still be visible. also seen this with many cars that had factory tape stripes. especially the firebird . those eagles would ghost back every time. this was lacquer then urethane clear.
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  3. #13
    Registered Member BamaNomad's Avatar
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    I need to do some research on this, but off the top of my head, I'm not sure I agree with CN's statement below...??

    "Guys have speculated about it being due to the hardness of the MIG weld. Hardness should not affect thermal expansion."

    ?? I think checking the thermal expansion coefficients of various hardness steels would supply an answer, but I don't have a manual handy...?

  4. #14
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    From what I know, if it's carbon steel of any hardness (carbon content could vary), thermal expansion will be the same for all.

    Stainless steels can have different thermal expansion rates, but they will only vary in a fairly small range, unless it's a specialty alloy with small thermal expansion rate (and that may not even be "stainless steel" per se).

    Aluminum will have greater expansion rate, and the various alloys will vary, but again not by much.

    Again, I don't think thermal expansion rate is what's happening when you have a "ghost line", it's primarily joint stiffness, and maybe the amount of metal in a given location - I.e., a lap joint has roughly twice the metal as the surrounding panel. Also a lap joint is 4 times stiffer than a panel, or a butt welded panel.

    Remember too, that an adhesive bonded lap joint will also exhibit "ghost lines". So it's not just welded joints either.

  5. #15
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    Shine is right about the old pin stripes showing through. In the 1970s and 80s a lot of the big trucks had multi colors with pin stripes between the colors, and when a driver changed companies and got a new paint job, those stripes would show no matter how well it was stripped and prepped before paint.

  6. #16
    Registered Member chasracer's Avatar
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    I worked in a body shop doing mostly tear downs and some final assembly work. On the odd car I was taught some metal patching. The owner had started working in the business back in the 40's after the war so he was up in age. The one thing I remember he said about metal from this work lesson is that unless you can use a patch from the same era of car as the one you are working on, you can never get it perfect - the metals are different and that's just the way it is. But with a big smile, he also said that means repeat business too!
    Remember the "13"


  7. #17
    Registered Member chevynut's Avatar
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    Quote Originally Posted by chasracer View Post
    The one thing I remember he said about metal from this work lesson is that unless you can use a patch from the same era of car as the one you are working on, you can never get it perfect - the metals are different and that's just the way it is. But with a big smile, he also said that means repeat business too!
    I don't think it's necessary to get a patch from the same era car but there could be slight differences in metal composition that could change the thermal expansion rate slightly. I don't know how much of a difference in thermal expansion rate would start to show problems in normal use. I would never use stainless steel to patch mild steel panels but I'm sure it's been tried.
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  8. #18
    Registered Member chasracer's Avatar
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    Quote Originally Posted by chevynut View Post
    I don't think it's necessary to get a patch from the same era car but there could be slight differences in metal composition that could change the thermal expansion rate slightly. I don't know how much of a difference in thermal expansion rate would start to show problems in normal use. I would never use stainless steel to patch mild steel panels but I'm sure it's been tried.
    I believe what he was referring to is that over time, the metal used for automotive panels has changed dramatically. We're taking a 1950's metal that is totally different from the 4x12 sheet of patch metal and combining the two together. Each metal is going to have it's own properties. Then we add a high amount of heat, plus another filler material to combine them. Today's high strength steels are both thinner and different from most of the metal in our cars. I'm not sure we can expect a perfect application and the lap seam that started this discussion, in and of itself probably leads more to the ghost line than anything else.
    Remember the "13"


  9. #19
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    Low carbon steel was preferred then for body panels, as it is now for patch panels. If the steel composition had changed, so would the alloy designation. I don't think mild steel has changed a bit, let alone "dramatically".

    Also, as we all know from the current discussion on steel and aluminum tariffs, most of the mild steel these days is imported. And don't think for a minute that steel from 3rd world countries is "advanced". It's made there the way it was made in the US 75 years ago. That's why they are 3rd world and we aren't.

  10. #20
    Registered Member chevynut's Avatar
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    Quote Originally Posted by chasracer View Post
    I believe what he was referring to is that over time, the metal used for automotive panels has changed dramatically. We're taking a 1950's metal that is totally different from the 4x12 sheet of patch metal and combining the two together. Each metal is going to have it's own properties. Then we add a high amount of heat, plus another filler material to combine them. Today's high strength steels are both thinner and different from most of the metal in our cars. I'm not sure we can expect a perfect application and the lap seam that started this discussion, in and of itself probably leads more to the ghost line than anything else.
    Yes I agree that if you take a patch piece from a car from the 50's it's going to be different composition than metal taken from a car built today. Today's cars are made with thinner, stronger steel. I wouldn't think of using metal from a modern car for a simple patch panel.

    However, that doesn't mean you can't get metal of the same composition that they used in the 50's, which was probably a mild (low carbon) steel. Plain mild steel sheet you can get today is usually something like 1008 to 1020 steel (.08% to .20% carbon) and it's likely similar to what they used back then. I'm not sure but I doubt the thermal expansion of mild steels differs much.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

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