Floor pan patch- how to finish out the underside.

Discussion in 'Body Restoration' started by David79Z28, Jul 16, 2012.

  1. David79Z28

    David79Z28 Veteran Member Lifetime Gold Member

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    Oct 22, 2009
    Greenville, TX
    I have two new patches welded from the inside of the car. I overlapped the patch panels and the existing metal by 3/4" to 1".

    My plan is to tack weld a few spots on the underside of the car,
    use seam sealer to protect the seam, then paint.

    Any suggestionns, constructive critisim, pictures of what has worked for you in the past would be appreciated.

    I'm looking for what would be acceptable to see, from the underside of the car, when I am finished.

    Thanks.
     
  2. RDB123

    RDB123 Veteran Member Lifetime Gold Member

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    May 11, 2012
    Livingston, TN
    We used fiberglass resin and mats on all the seams from under neath. After they set up just sand them down to get a feathered edge. Then prime and paint.
    [​IMG]
     
    Last edited: Jul 17, 2012
  3. TX79Z28

    TX79Z28 Veteran Member Lifetime Gold Member

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    May 3, 2007
    San Antonio, TX
    David,

    I am no expert, but I have done my own floor boards and experimented with different techniques. I can't say one or another is "wrong" per se, but will give you my opinion.

    People will say that it's best to butt weld them, that may be true, but my skill is not there to do that. So, like you, I went the lap weld route. After fitting my panels and securing them VERY WELL to the braces and to the existing metal with zinc sheet metal screws I tack welded top and bottom, until I got the tack welds probably an inch apart. Then I connected them with a bead, to make a continous weld around the whole panel. I grinded the welds down. After I shoot the epoxy primer, I will use filler to "smooth" the area and blend it in.

    I used a light behind the seams to find all the pin holes where I missed a spot and spot welded them, so by doing both sides, and smoothing with filler, you would have to really try to find the repair. I did try the seam sealer route, and personally I think it looked like crap and was NOT happy with it.

    When I get done, I either remove the screws and fill them by welding, or cut off the excess, grind it smooth, and follow with a spot weld over it, and grind it smooth again.

    FYI, if you lap weld, make sure you use weld-thru primer on both surfaces that are overlapping.

    Hope this helps.
     
    Last edited: Jul 17, 2012
  4. CDesperado

    CDesperado Veteran Member Lifetime Gold Member

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    Oct 1, 2004
    Dallas Texas
    I think the reason recommend butt welds is that if you overlap the panels, it creates a "pocket" where the overlapping metal can shift, rub, and eventually begin to rust. Does the weld-through primer will help with that ?
     
  5. TX79Z28

    TX79Z28 Veteran Member Lifetime Gold Member

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    May 3, 2007
    San Antonio, TX
    True, I can see where the "pocket" could develop rust. In theory, the weld thru primer is supposed to stop it. That's another reason I make sure the overlap areas are SUPER tight without any gaps. Also, I do all the fitting before the primer, and only spray it just before the final fitting.
     
  6. Rich Schmidt

    Rich Schmidt Veteran Member

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    Mar 27, 2010
    Denville NJ
    Go under the car and look at where the welds are from underneath(you can see them by the discoloration of the metal). Carefully cut off as much of the excess as you can beyond the weld,grind the edge of whats left as smooth as you can,get paint inbetween any part of the seam thats left,then decide if you want to paint or undercoat it ect.
     
  7. 1971CamaroGuy

    1971CamaroGuy LS Swapped 1971 Camaro

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    Oct 31, 2004
    Arkansas
    I know people talk about the rust in between lap welds....but seriously if your not lapping bare steel and driving through rain storms for the rest of the cars life....why is there for concern?

    These cars were built with lap welds all over it and I have yet to see a factory lap weld fall apart due to rust before a flat section next to it

    A lap weld would be the ideal way (structurally) to repair a floor it would seem.
     
  8. David79Z28

    David79Z28 Veteran Member Lifetime Gold Member

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    Oct 22, 2009
    Greenville, TX
    Thanks everyone. You have given me some great tips.

    Hopefully I can get sometime this weekend to work on it.
     
  9. TX79Z28

    TX79Z28 Veteran Member Lifetime Gold Member

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    May 3, 2007
    San Antonio, TX

    Completely agree. Personally, I chose to lap the panels and have a continous weld bead on both top and bottom side of the panel. I think this, along with weld thru primer in between is more than sufficient
     
  10. ATM

    ATM Veteran Member Lifetime Gold Member

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    Oct 10, 2009
    Morris, Illinois
    I keep on hearing that they don't even weld patches all the time any more, they epoxy them on or something. It would make me nervous on a floor though which I would imagine should be structural. I overlapped mine as per recommendation of my FIL, who is a bodyman by trade for over 40 years. That is not to say that if I had the skills to do it differently I would not have butt welded, because I would have, but I don't weld.
     

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