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Thread: Minimum thickness for filler over weld

  1. #1
    Join Date
    Oct 2007

    Default Minimum thickness for filler over weld

    Hi all! First time posting here but have been member for 17 years. Have no meaningful experience with bodywork or painting so be gentle with me. Am restoring a 1970 Plymouth Duster and have replaced the lower rear quarters behind the wheekwells. Butt welded on with MIG and after grinding down they look not too bad, only a few tiny surface inclusions, on the finished face. Did my best to control the heat distortion and think I did pretty good. Each side is a straight 3 foot long weld. After checking with straight edge and feeler gauge along where the bead was, I can find no depression form weld shrinkage that is more than .010". Researching on the forums seems to indicate that a layer of short strand glass filler on both sides of the weld is a good idea to seal out any potential moisture. My question is whether or not to use filler on the finished side of the weld or just go with a wipe of epoxy primer to fill the weld inclusions then heavy coat or 2 of epoxy primer to fill the depression. I'm wondering at what point is it not practical for apply filler vs using primer to bring up a very small low spot. Thanks for any help!


  2. #2
    Join Date
    Nov 2005


    Hi Brian
    A lot of folks here may have their own approach with that situation so I'll tell you mine. We don't use fiberglass filler on the backside of the repair, we use a good seam sealer. Why? Because the seam sealer does a better job at "sealing" the seam from moisture than filler. However most repairs of that nature are difficult to get behind and need to be addressed in front and that's where a fiberglass filler works best.

    We normally press a thin layer of the fiberglass onto the surface then sand it prior to the application of polyester filler. As long as the metal is abraded/sanded there is no minimum thickness of the filler, just be sure to press the fillers into the scratches, that's what makes it bond well with the surface.

  3. #3
    Join Date
    Nov 2013


    the whole reason for short strand glass is to seal the surface. that filler is waterproof and is a good first pass over the weld area. i like lens method of using seam sealer on the back side, no reason for filler there.
    but if you don't need any filler, go straight to epoxy to seal the front. then pat yourself on the back for doing a weld repair that didn't need filler.
    b marler

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