Register now to get rid of these ads!

Technical Butt weld or overlap when welding in floorpans?

Discussion in 'The Hokey Ass Message Board' started by evintho, Jan 20, 2020.

  1. anthony myrick
    Joined: Sep 4, 2009
    Posts: 8,653

    anthony myrick

    Working at the same body shop for 13 years I got to see some of the same cars.
    Meaning, I got to repair several cars that we had already fixed.
    I replaced a rear body in a car that we had previously replaced 3 years earlier. Our standard operating procedure for repair always included weld thru primer, replacement of seam sealers and undercoatings and we etched and primed all panels as if it were an exterior panel. In other words, we didn’t seam seal or undercoat over bare metal or e-coat.
    The rear body weld joints were as clean and rust free as a new car.
    The lap joint is not the enemy, lack of preparation is. Even a but weld can rust if left to the elements.
    Last edited: Jan 20, 2020
  2. gene-koning
    Joined: Oct 28, 2016
    Posts: 2,549


    I live in the rust belt. For 25 years I've lap welded in floor pans, the few comebacks were when the original metal rotted away beyond where the new pan was installed many years later, and those were on snow plow trucks. The weld area was as clean as the day it left my shop the first time. When done right, there really isn't much difference between lap or butt welding 10-15 years later. Its the preparation before the job, and the sealing after the job that makes the difference.
    For the record, 90% of the rusted floor pans rusted out from inside the car, not from under it, unless it was sitting in water or on the floor pan. Those leaking window and door seals and the water drug into the car on your feet that soaks into the carpet is what does in most of the floor pans. Gene
  3. 6inarow
    Joined: Jan 24, 2007
    Posts: 2,326


    as said earlier what a GREAT way to perfect your butt weld technique
    49ratfink likes this.
  4. dan31
    Joined: Jul 3, 2011
    Posts: 1,086


    Can anyone give an example of a floor panel that is butt welded from the factory?. I'm sure there may have been some , but can't think of any. Floors rust out from water coming off of your shoes or seals leaking. Most of the rust starts right under the driver's foot area where there most likely isn't even a joint.
  5. Pats55
    Joined: Apr 29, 2013
    Posts: 523

    from NJ

    You cannot create rust without oxygen. Whether you butt weld or lap weld prep everything thoroughly with a metal prep that contains zinc that doesn't require a water rinse. You can use your weld through primer and encapsulate everything in the repair area with two coats of the moisture cured aluminum pigmented permanent rust sealer. You then can prime and paint your car and get many years out of your repair Been doing it this way for 30 years
  6. 49ratfink
    Joined: Feb 8, 2004
    Posts: 18,113

    from California

    we are not building a car from fresh stamped parts at the factory. in my experience floor pans are mostly spot welded. we are patching a floor that 9 times out of 10 will be one giant stamped piece. no comparison to what is done at the factory.

    I don't think anyone would disagree that butt welded patches are the way to go be it body or floor. with that said do it however you want.
  7. jvo
    Joined: Nov 11, 2008
    Posts: 228


    I do both. Lapped where the factory did it, but welded where I'm replacing metal where the factory did not lap it. I also use weld thru primer on everything. I grind the weld top and bottom so it looks like factory. I even metal finish it to some degree. It doesn't take much more time to do it so it doesn't look like dog shit.
    Biggest no no for me, is welding right in a corner. Looks like dog shit and you have a really hard time grinding the weld. Yeah, nobody sees it, but it still looks like dog shit.
    I always take a hammer and dolly and turn an edge so I have a weld about an inch or so away from a corner, then butt weld it.
    And yes, I don't like dog shit, but I'm not very fond of dogs either. There. All you dog lovers can call me a dickhead and I won't even care.
    Lloyd's paint & glass likes this.
  8. 19Fordy
    Joined: May 17, 2003
    Posts: 7,298


    I over lapped my floor pans and trunk floor about 1 1/2 inches and welded them in from both top and bottom.
    Used weld thru primer on overlap.
    Then ground down both welds (top and bottom) with a die grinder. No seam sealer.
    Lots of work but, I wanted that extra thickness at the perimeter. Metal was 16 ga.

    Attached Files:

  9. dan31
    Joined: Jul 3, 2011
    Posts: 1,086


    Your correct ,most floor panels are spot welded together but it's on a the lap joint ,right? , must be ,'cause you can't butt material and do a spot weld can you?. Many cars do have one piece floors, many more have front and back sections. Pretty much a direct comparison . Pretty much every car ever made uses lap ,butt and spot welds from the factory. Most floors have lap joints and that's what there talking about here. Butt joints are better on the body because you may not get a " witness line " from the difference in material thickness . It's also been said about rust with lap or flanged joints bit what if you can't get to the back of the butt weld to dress it ? , will you get rust then? , I bet you will. So are you better off using a flanged weld with weld through on say a driver ?, or maybe even panel bond so no rust issues at all but maybe a possible " witness line". I don't use a hammer when I need a saw.
  10. 49ratfink
    Joined: Feb 8, 2004
    Posts: 18,113

    from California

    Your correct ,most floor panels are spot welded together but it's on a the lap joint ,right? , must be ,'cause you can't butt material and do a spot weld can you?.:confused::confused::confused::confused::confused::confused::confused:
    anthony myrick likes this.
  11. southcross2631
    Joined: Jan 20, 2013
    Posts: 4,414


    If you decide to lap weld. Use a flanger puncher to pop some holes in your new panel then plunge weld the holes. Use weld through primer on both the old metal and the new panel. then weld both top and bottom . Use seam sealer both top and bottom to seal from moisture .
    Or just butt weld the panels 2012-01-09 00.59.47.jpg and seal them . Floors butt welded. Spare tire well lap welded.
    joel and Lloyd's paint & glass like this.
  12. earlymopar
    Joined: Feb 26, 2007
    Posts: 1,381


    While I've done both lap and butt joints on floor panels, I have given thought to using the same method as you used. It does add some to grinding time but the install time is much better with this method versus all of the time spent fitting a panel for butt welding.

    - EM
  13. I Agree with both of these guys! Back in Nov. of '73, I bought a brand new '74 CJ-5. (Remember when you used to be able to do that?) I lived in the same part of the country as Gene and I faithfully washed that thing every week. Even though it was undercoated, I probably kept it wet and didn't realize it. By spring the floor pans were rotted out!

    I carefully made and welded in new floor pans and thought I was good to go. In a couple of years I had to do it again. I eventually had to buy a new fiberglass body.

    If I had to do it over again, I would just overlap it and use the panel glue and let it go. That stuff is much stronger, cheaper and faster than welding. Like the Shift Wizard says, "You'd be done by noon!" I think you guys are over thinking this.
  14. Let’s just say you’re going to get a factory style reproduction panel,,, you’re going to remove the old one end to end and LAP weld the joints. Probably plug weld drilled holes so it’s like spot welds.

    But -
    Let’s say that you only need part of the factory replacement panel,,, because the intricate part of your floor pan with all the shape and change and beads is rotted out. Then you cut the patch needed from the replacement panel and BUTT weld it in, finish it nice and nobody can tell you were ever there.

    Sometimes the patch you cut from the replacement panel lands over part of the factory LAP joint. So you need to BUTT weld part of it and then LAP weld part of it.

    Basically it’s a floor and that’s a good place to practice. And obviously it Don’t really matter how you might do it as long as it fits well and does not leave jagged end and cavernous gaps on the underside. GREAT PLACE TO Test your apprentice’s ability is on the floor.
    49ratfink and dan31 like this.
  15. I can't speak to how every floor in every car was originally manufactured and I won't condemn the use of lap welds in every application. What I will say is if you are building a nice car and trying to do a quality repair, throwing a set of lap welded floor pans into the car with a line of seam sealer around them ain't gonna get you there. It may last but it screams amateur and looks like a bucket of smashed assholes if somebody looks underneath the car. I don't thing most of us are trying to half ass salt trucks back together for the next plow season, why do that kind of work on your pride and joy?
  16. Budget36
    Joined: Nov 29, 2014
    Posts: 7,229


    Hey Lloyd,

    Is that brush on? I couldn't tell from your pic.
  17. No it's a spray bomb. I normally use the 3M but there wasn't any on the shelf the last time i bought it. But the SEM seems good. Takes a little longer to dry before you can weld. It'll let you know really fast if it isn't dry lol
    Budget36 likes this.
  18. evintho
    Joined: May 28, 2007
    Posts: 1,833


    Thanks for all the insight everyone! I've decided I'm going to butt weld the rest of the floorpans. I'm actually getting better at it. Practice makes perfect!
  19. no55mad
    Joined: Dec 15, 2006
    Posts: 1,891


    Probably not much oxy/acet use going on anymore but brazing used to be an option.
  20. David Coleman
    Joined: Oct 15, 2019
    Posts: 29

    David Coleman

    I think me having a life-long skill of oxy-acetylene welding sheet metal, was a blessing when I bought my Miller Syncrowave 250 TIG welder. Very easy learning curve getting to use the TIG torch, like I use/used the gas torch. Remember, the first airplanes were gas welded together. As an aside: how many of you started welding/brazing (I was 13) with a $15.00 carbon arc torch set from Pep Boys? And by the way, Bronze rod should be in your rod rack as well as Brass rod, if you are still doing gas welding/"brazing." Another aside: I think back to an old timer that was jerking me around (took me years to discover it). He said the way to get beautiful, wide gas welds on thin flat sheets and 18 gauge steel tubing, was to use a big tip (like a #1 Victor) and turn the gas way down for a nice gentle flame. That IMHO is a guarantee to get a backfire, and to spray the melted puddle all across the room.
    Montana1 likes this.
  21. Most WW2 era planes made from aluminum were gas welded since the TIG system wasn’t perfected and patented until 1941.

    Funny thing is today apparently aluminum can’t be welded without an AC high freq computer controlled pulse tig or a messy mig process.
    anthony myrick, indyjps and joel like this.
  22. Andy K
    Joined: Jan 12, 2020
    Posts: 34

    Andy K

    My OCD heavily agrees with your OCD

    Sent from my iPhone using The H.A.M.B. mobile app
    anthony myrick likes this.
  23. Heck, I learned to weld sheet metal with Oxy. Acet. at the age of 14. I even used a mini stick welder back in the day. It was only recently that I have obtained a TIG and MIG because of the new compact units available. I still prefer Oxy Acet. for many jobs. ;)
    pitman and dirty old man like this.
  24. fordcoupeguy
    Joined: Apr 26, 2014
    Posts: 164


    Ive seen 70+year old patches brazed on to truck floors still as sold as the day they were installed. Why has that method fallen out of favour? If you have ever tried to remove one, you might consider going back.
    dirty old man likes this.
  25. 34Larry
    Joined: Apr 25, 2011
    Posts: 1,649


    I was an iron worker for 30 years fitting and welding sheet, plate, shapes, you name it. Lap welding sheet correctly requires twice the work as butt welding if your doing it right, (weld one side, turn it over weld the other side), warping is the culprit doing this. I much prefer butt welds.
    I had to move my drive line tunnel when bringing the body down because of the offset of the Jag pumpkin and butt welding was the ticket. 100_1414.jpg
  26. chev34ute
    Joined: Nov 13, 2011
    Posts: 1,110


    I was told be a very experienced rodder who has run a rod shop for decades that butt welding creates too much distortion. At the same time, I have been butt welding patch panels onto my Model A Roadster Quarter panel with next to no distortion. I always believed a lap weld would make a panel stronger due to the overlapping sheet metal, but apparently the opposite is the case. I am so confused.

  27. Keep that away from those birds
  28. anthony myrick
    Joined: Sep 4, 2009
    Posts: 8,653

    anthony myrick

    You don’t need to overlap a patch to make it stronger. Never understood the logic in the argument that an exterior piece of sheetmetal needed to be stronger than the factory made it.
    Overlapping exterior panels such as patch panels or repair panels in non factory locations is only for speed or lack of skills.
    All welding creates distortion.
    A lap welded panel doubles the thickness at the join and is harder to remove distortion.
    When I was first certified for collision work I was instructed to make sleeves or lap weld all section locations. Then I started working at a shop that had factory repair manuals for the cars the shop was factory certified to repair.
    Mercedes, Lexus, Volvo, Porsche......There was ZERO lap welding of section locations or sleeves recommended by the manufacturer. Not even a frame rail section.(uni-body)
    I haven’t overlapped a seam (other than factory ones) in almost 30 years.
    A properly fitted and welded butt joint takes less weld, less heat, less grinding, less distortion, less hammer and dolly work and less filler work.
    Last edited: May 25, 2020
  29. southcross2631
    Joined: Jan 20, 2013
    Posts: 4,414


    I just finished the floors in my 65 Comet SEGA super Stock and I did a combo of butt welds and lap welds. The only water it will see is the water box and when I have to tow in the rain. So the lapped seams will get Flex Seal on them and rustoleum black paint.
    If somebody wants to crawl under the car to see if it is butt welded or lap welded they are either helping me pull the transmission or they want to buy the car.
    I will post some picture of the bottom side of the car when I get it sealed and painted. DSCF3646.JPG
    '51 Norm, GordonC and shivasdad like this.

Share This Page

Register now to get rid of these ads!


Copyright © 1995-2021 The Jalopy Journal: Steal our stuff, we'll kick your teeth in. Terms of Service. Privacy Policy.

Atomic Industry
Forum software by XenForo™ ©2010-2014 XenForo Ltd.