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Discussion Starter · #1 · (Edited)
I'm just thinking out loud here, and I appreciate your opinions.


Lap Vs Butt joints when installing floorpans.


I hear everybody's concerns over the possibility of internal rust when doing a lap joint, but I found several lap joints on my rocker panel covers when cutting them off my 57. These were done between 1976 and 1984 and nothing was done to treat the two panels with any of the rust eating chemicals or coverings like POR. When I cut them apart, all I saw was some surface rust ... certianly nothing to weaken the welded areas.

There are products available now that weren't available 20 years ago, like the weld-through primers and epoxy primers, POR and Rust Bullet that should completly seal off any possibility of mosture getting into the joint.

So, I guess what I'm asking is why are butt welds are prefered over lap ?

Is it for looks or for "correctness" ?

Does a lap joint make the car less desirable during a sale , and if properiy covered, who could tell or know ?

I just want to put this car on the road in the safest possible condition, and it seems to me that a lap joint would be a stronger joint for a novice welder.
 

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Its cosmeticI weld the outside of the body and leave the laps inside. There are more products than Rust Zero and por-15 to seal laps. Go to your local Paint supply house and get Lap Seale4r . I agree they look crappy if its going through Barret Jackson but it is an affordable street driven car Screw Barret Jackson they don't need to look at it.:D
 

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Hi Guys
Dave thanks for the kind words.:eek: Let me put it this way most of you are performing the repairs on your own cars and at that point it is up to you to choose what works best for you :) and Mokicruz had a great point. My concerns are the following : One when it comes to metal finishing I can not have the lap joint in my way, I need to get on the weld from both sides and work the heat affected zone (HAZ) to properly relieve the area that has been shrunk and distorted from the weld. Once I have have accomplished that I can properly metal finish the area so the customer is unable to detect the welded area.
A lap joint is fine if protected and will not rust for years if done properly, and care is taken during fitment. The only caution I have is when the area lapped over is painted and introduced to direct sun light when the panel reaches a temperature were the metal expands the thickend area will move differently
and show some distortions , sometimes not even noticeable but there. Agian this may seem extreme but if I cover all the bases you have more information to work with. If I tell you 10 great ways to perform metal work and only 8 work best for you , I'm a happy guy :)
 

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Discussion Starter · #5 · (Edited)
Thank you Steve.

Your explanation makes sense for outside body panels, and that will be how I will repair the area behind the B piller to the rear wheel opening.

I would like to ask another question concerning butt welding , and if you need to edit and move it to a different thread, please feel free.

I see clips that space and hold the two panels .040 apart sold by Eastwood ( I think )

I can see where the "extra metal" to fill the gap is provided by a Mig,
but would you use the same gap if you were going to Tig ?

And what about using the heat dam stuff that is supposed to prevent a large heat affected area. Does that change how close you butt up the two panels ?
 

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Hi Bama57
Great questions, here is an illistration to give you a better idea on the use of butt clamps.

Notice the gap in the replacement panel were the b-clamp fits nicely and allows you to tighten the gap in both panels.
Now if I'm mig welding I take 3 to 4 short burst with the welder and move on to the other end of the panel, after waiting a minute or so depending on the lenth of the panel it self, I run another 3 to 4 short burst.
I at one time use to think if you ran a small gap between the panels it would reduce the warpage. But I think it may cause a bit more. Also try not to use a blow gun once in a while on the welded area you may see that letting it cool down on its own will create less warp. As far as heat block that will work but you can contain it with care as well. What little heat distortion you do get would come back out once you relieve the affedcted area.
 
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