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Proper prep before final Welding

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Hi there,  this is for my e21 floor repair, but will apply to my 2002 in the future.  This is also my first time doing any metal repair, so hoping to do the right thing the first time.  Might be a simple question, but i figured i'd ask now, cause once its welded in, well....


I am getting ready to weld in a new floor pan ( W&N) and had some questions on how to properly prep the panels before  plug welding it in.  The replacement panel has an e-coat or primer applied and I'm trying to figure out what to do to the mating surfaces in the area of the plug welds.  The sides i'm welding to ( frame rail and along firewall) are bare metal.  I used por-15 down the center of the center of the frame rail as I figured it wouldn't be getting too hot.  



QUESTION: Should I clean the e-coat off the new pan in the strips to be welded and spray Weld Thru Primer so its the same on both mating faces? Does it matter?  I've read that Weld thru primer doesn't hold well to paint (isn't really a primer but a coating) so didn't think it made sense to spray it over the e-coat.  Something better to use?   How far around each weld should the paint me cleaned off?


The plan after welding was to put down eastwood rust encapsulator, seam seal, then paint.  


Any thoughts or best practices on proper prep for the plug weld zones?





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34 minutes ago, floatinghead said:

The plan after welding was to put down eastwood rust encapsulator, seam seal, then paint.  


I would weld bare clean metal, clean it back an inch or so.If you follow thru with the above plan no need for weld thru primer.

I'm assuming that "plug weld" means "spot weld"? 

Fit it all together and fix it all in placedrill some holes from pan to frame rail every 6 in. or so. Do you have clecos? Do the edges too,

If not use screws and nuts to draw the pan and rail together, monkey with it until you have full contact all around(best possible) 

Tack weld it all together before concentrating on any one area, helps control distortion,


Be certain your spot welds  actually engage the floor pan.

I've seen repairs where some of the spot welds just fill the holes and dont actually fuse with the floor pan metal. Nice prep, looking good.

Edited by tech71
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I did weld through on all surfaces and encapsulater in the lower part of the frame rail. Anywhere metal is sandwiched I removed all previous coating, took down to clean metal, then a layer or two of weld through….

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I do probably need to add some more holes for bringing the panels together for the spot welds.  I did use cleco's and they were super helpful as i was getting everything lined up and cut.  I usually run a few mockup panels as well to get my process a bit more dialed before i weld.


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I don't like the weld- through coatings.  They're designed for a 400a stick weld, not an 80a MIG.


What I have come down to in the recent past is clean metal for welding (prep the inside of the frame rail

to the 9's otherwise, but leave the flange clean) then fog the darned thing with a waxy filler.

3m cavity wax, waxoyl, something like that. Lots of it, to really seal the weld.


I cut up an E46 the other month, and the pinch welds were electrocoated full of primer-

those baths they run the shells through really do work.  A wicking rust preventative is the closest

you and I can get, unless we buy a swimming pool...




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