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Unread 02-16-2011, 06:11 PM   #1
Ken4444
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Sheetmetal repair: joint question

I will be welding in new sheetmetal in the usual rust spots in my '85. I am torn between using a butt joint versus a lap joint. Right now, I'm thinking I will use a lap joint with a flanged edge, at least for the larger areas. Am I right in assuming that I should use seam sealer with a lap joint? If so, I'm not sure how that will interfere with the spot welds. Also, what gauge sheet metal should I use to repair the floor pan holes?

Any suggestions or advice would be appreciated!

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Unread 02-16-2011, 06:38 PM   #2
Kettles
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I would cut out the spots to have nice clean straight edges and make a patch panel to fit as best as possible, and weld with but joints. The lap joint would need to be sealed (or welded) on the back side to prevent stuff from sitting getting in the overlap and rusting again. The butt joint will look better too, although on a floor pan it might not be a big deal to you.

i haven't done any automotive sheet metal repair myself so I don't know what gauge floors tend to be. Someone else knows I am sure.
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Unread 02-16-2011, 08:48 PM   #3
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It will be easier to make a good looking repair with minimal distortion using a lap joint with a joggled seam. I would goop the hell out of the inside of the joint using POR-15 and then maybe seam sealer, undercoat, or even something like Herculiner if the area is exposed to road spray. Finish the exposed side of the seam using standard autobody filler. The tighter and flatter your welded joint, the less filler you will need, which is a good thing. You can buy either an air powered or hand tool that will form the joggle you will need.
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Unread 02-17-2011, 06:32 AM   #4
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16 gauge or at the lightest 24 gauge for floor pans,i would go with 16 gauge.

Netpackrat pretty summed it up well,a lap joint will help keep distortion down to a minimum and being floor pans it would very easy to prep and seal underneath,if repairing a body panel,you are most likely never going to get in behind it to seal it up from weather, so a lap joint or butt weld really makes no difference as far as one holding up better then the other from rust,they will both be exposed metal/welds moisture behind the panel.

Which ever technique you use,since you had to ask about it,then i am assuming you have minimum expereince doing this,so make sure before you do ANY welding on the actual panel to have some scrap to mock up the same kind of joint you will be using and practice welding to set up your machine and see how to tack weld it all together at best for a good solid result with minimum warpage.

You can buy a pnuematic flanging tool at alot of stores and online..harbor freight..eastwood online...etc to create a near perfect flange on the edge of you'r repair panel
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Unread 02-17-2011, 06:50 AM   #5
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Ken I would try and avoid the flanged repair. It does make a nice joint and will not be as prone to warp but it's a place for moisture to get trapped and rust to from. Seemsealer will help allot in taking care of that but the fact remains that it could get damaged and allow water to get in there. I'd go with a butt joint. In fitting your patch panels make a template out of stiff paper of the area that needs fixing then transfer that to the patch. It's easy enough to cut the patch a little big and trim the tub so that you have a good fit.

16 gauge will work great for what you'll be doing.
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Unread 02-17-2011, 07:55 AM   #6
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You're best bet is to go to a few of these forums where the Pro's who do it day in and day out to get opinions and facts to the different techniques and reasonings behind the techiniques and welding processes to use to best suite your repair.

http://www.metalmeet.com/forum/index.php?

http://www.tinmantech.com/

http://www.thesheetmetalshop.com/forum/

Ask them the reasoning behind using a flanged joint,overlap and butt joint and end results and which is better for you're case..also ask the reasonings behind the different welding processes..TIG..MIG..Fluxcore..Brazing...etc,,they all have their ups n downs and none of them are actualy perfect..no matter what technique and welding process you use..there WILL be warpage and body work needed afterwards if welding an external patch panel that is exposed to visual sight afterwards.

Also ask them if a flanged joint will actualy make much of a long term difference in rusting out compared to a butt joint..i think you're answers will be real surprising when the pro's jump in on the hands on expereinces they've had with it over the years.

There is also a joint sealer/glue out that has proven to last just as long if not longer than a welded sheetmetal joint in patch repairs now..and alot of auto body shops have gone to that method because there is minumum body work needed after it is done..and the glue is near as strong as a welded joint..AND takes ALOT less time to do the repair.

I have a 1969 El Camino project i will be starting on soon to get done and sell for a profit,one of the rear quarters has a pretty serious dent and i'll be buying a full 1/4 replacement panel for it..and have decided to go with the panel adhesive for it over TIG or MIG welding it up like i usualy would, because this panel advhesive has proven over many years to be very durable and easy to use over welding them in now a days...take a close look at most of the vehicles built in the past 10 or more years..there is more and more panels being glued in instead of welded because this stuff has proved itself very durable and useful when used properly.The panel adhesive bonds and seals all in one step..win win situation.
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Unread 02-17-2011, 02:26 PM   #7
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If you are going to do a lap joint you don't have to weld it. You can "bond" it with new compounds that make it as strong as the steel. Check with local body shop supply.
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Unread 02-17-2011, 03:24 PM   #8
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Originally Posted by Ironworker709 View Post
There is also a joint sealer/glue out that has proven to last just as long if not longer than a welded sheetmetal joint in patch repairs now..and alot of auto body shops have gone to that method because there is minumum body work needed after it is done..and the glue is near as strong as a welded joint..AND takes ALOT less time to do the repair.

I have a 1969 El Camino project i will be starting on soon to get done and sell for a profit,one of the rear quarters has a pretty serious dent and i'll be buying a full 1/4 replacement panel for it..and have decided to go with the panel adhesive for it over TIG or MIG welding it up like i usualy would, because this panel advhesive has proven over many years to be very durable and easy to use over welding them in now a days...take a close look at most of the vehicles built in the past 10 or more years..there is more and more panels being glued in instead of welded because this stuff has proved itself very durable and useful when used properly.The panel adhesive bonds and seals all in one step..win win situation.
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Originally Posted by baldfatdad View Post
If you are going to do a lap joint you don't have to weld it. You can "bond" it with new compounds that make it as strong as the steel. Check with local body shop supply.
Its called Panel Adhesive..

3M..SEM products..MasterBond all make it along with some others and can find it online or an auto body shop supply store...its cheaper to buy online.

Do a google search on "Panel Adhesive" and you'll find all the types and brands along with all the people and shops who use it and how well it holds up and ease of use,,,

And this is all coming from someone who has always been a firm beleiver the best way to bond any steel is to weld it..but this stuff has proved itself to perform well in sheetmetal

This is a GREAT alternative for anyone who doest have a welder or the expereince to confidently weld up a panel
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