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How much space around weld area should be weld-thru primed?

As the subject says, I'm wondering how much space around the weld area should have weld thru primer? For example, when preparing the back side of a part that will be but welded in, but not accessible for priming after welding, I've been putting down epoxy but taping off the but weld edge with 1 inch tape. Then putting weld thru where the 1 inch tape was, then welding. Is this the correct procedure? Or should I just be putting epoxy on the whole piece and what burns away, burns away?
Old 07-10-2017, 03:21 AM
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As the subject says, I'm wondering how much space around the weld area should have weld thru primer? For example, when preparing the back side of a part that will be but welded in, but not accessible for priming after welding, I've been putting down epoxy but taping off the but weld edge with 1 inch tape. Then putting weld thru where the 1 inch tape was, then welding. Is this the correct procedure? Or should I just be putting epoxy on the whole piece and what burns away, burns away?
It's hard to answer it scientifically as I don't fully understand it but what I can tell you is so far a handful of OEMS have started to recommend to do exactly that. Epoxy prime everything and what burns burns. Come back after all other refinishing and apply cavity wax to any areas that you can get inside of. Any flanges or overlapping metal we "finger" seam sealer into the edge of the flange after primer in case there are any hairline or larger gaps. You will not see this seam sealer as it is only wiped into the crevices and excess is removed.
Old 07-10-2017, 05:37 AM
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Hmmm...very interesting. So, does his apply to overlapping sleeves that will be plug welded? In other words, with two overlapping sheets of metal where one is being plug welded over the other to increase strength, just epoxy both mating surfaces and simply grind away the epoxy where the plug holes are for welding (so any burned epoxy on the heat affected zone of the plug has no primer? Or is this a scenario where a 1 inch area around each plug should have weld thru primer? If the former, is there any use anymore for weld thru primer?
Old 07-10-2017, 07:21 AM
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This one has thrown me for a bit of a loop. I have some parts that I epoxy primed leaving the flanges for weld thru. I can put on another coat of epoxy to get the flanges covered and follow the method that Nathan is saying some are now using. I watched a couple of youtube videos where people tested weld thru primers and their conclusions seemed to be that it's not very useful, so I can see why there may be a movement away from it..... Just not sure how best to proceed. If there's any further info out there, or other opinions, I'd appreciate hearing about it.
Old 07-11-2017, 04:06 PM
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It is confusing to say the least. Some OEMS are still recommending it but they are consistent in saying that they do not want weld-thru primer in the actual location of the weld, just surrounding the weld area as best as possible. My advice is just epoxy everything and only remove primer if/when it interferes with the welding process in as minimal area as possible.
Old 07-11-2017, 05:46 PM
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In a way that makes it easier because there's a lot less masking involved. One new problem it creates is how best to remove the epoxy from the bottom piece where the plug weld will go. Weld thru just scratches off but epoxy not so much.
Old 07-11-2017, 06:14 PM
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In a way that makes it easier because there's a lot less masking involved. One new problem it creates is how best to remove the epoxy from the bottom piece where the plug weld will go. Weld thru just scratches off but epoxy not so much.
Use a dremel with a burring tool or like a spot weld drill bit
Old 07-12-2017, 08:17 AM
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Interesting discussion, which I've also been pondering. The weld-thru I've used before will fall off if you just look at it the wrong way. I like the idea of slapping epoxy on everything and just removing it in the exact areas where you're welding. Let us know how you get on with this, please!
Old 07-12-2017, 02:50 PM
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we have talked bout weld primers many times in the past .
it's not a good idea to just use epoxy or a etch primer and think it's going to protect any areas were it is not applied or gets burned off from your welding .

a weld primer is zinc based . being zinc it will weld and also protect the area around were the primer is put .
most people that have problems with weld thru primers have the problem because of a few reasons .
the 1st reason tends to be they use to much of it and it will splatter your weld and you will go thru hell trying to get good welds .
all you need is one very very thin coat of the weld primer for it to do it's job . remember it's zinc !
zinc doesn't have to over bare metal to protect the bare metal it just has to be close to it .

2nd problem is most people do not wait long enough for the weld thru primer to dry all the way before they start to weld . some times the wait time is 6 to 12 hours before you should try and weld or again you will get lots of weld splatter as your burning the solvents trying to do your welding .

there is another posting i did show the way to use weld primer and how to weld when your using it . i will look for it and re post it .

when it comes to products like cavity wax they are not all to good for long time rust thru protection . they work good for things like pinch weld areas but not the best for protecting back sides of bare metal welded areas .

if your weld thru primer is flaking off then you have not cleaned the bare metal or sanded the bare metal or you applied way way to much or the weld thru primer .

Last edited by 962porsche; 07-13-2017 at 06:34 AM..
Old 07-13-2017, 06:26 AM
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Thanks 962! If you find the link to your instructions please do repost.
Old 07-13-2017, 06:43 AM
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I'd be interested to read the OEM documents supporting the epoxy primer technique.
Old 07-13-2017, 08:51 AM
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I'd be interested to read the OEM documents supporting the epoxy primer technique.
Not what you asked for but this is Hondas position on weld-thru. They do recommend it for Squeeze type resistance spot welding but not any other type.

"Weld-through primer should not be used when doing MAG plug, MAG butt, or MIG Brazing. Further study has shown that weld-through primers can negatively affect weld or joint quality."
"When doing MAG plug and butt welding, or MIG brazing, remove only enough of the factory e-coat to allow bare metal in the weld or joint area. Then, apply corrosion-inhibiting primer over the completed weld or joint."
Old 07-13-2017, 08:58 AM
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Here's Fiat.
"Fiat Chrysler Automotive (FCA) recommends and where this information can be found.

FCA has recently updated their weld-through primer recommendations. Previously written manuals by Fiat recommended the use of weld-through primer, while Chrysler written manuals advised against the use of weld-through primer. The newly updated editions of all FCA manuals forbid the use of weld-through primer. However, you may find that currently on the free site that some manuals have not yet been updated and still recommend using weld-through primer. Per our contact at FCA, the free site is still in the process of being updated and weld-through primer should not be used."

Keep in mind up until recently every manufacture thought this stuff was absolutely necessary but it appears little by little that position is changing as manufacturers have started to see cases of significant failure.
Old 07-13-2017, 09:04 AM
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i can't find the write up i did but i did come across .
another posting with weld primer on page 2
question for 962 on welding flares is the title .

when it comes to weld primers there has been the debate for years .
some techs like using it others dont .
the write up I-car did about fiat and chry. is the same debate .

what it seams to always come down to is the knowledge of uses and when to use it and when not to .
one problem is people tend to use way to much weld primer . this can and will contaminate your weld . this is why some only want you to use weld primers with a resistance weld .
i have said this countless time many people think if a little is good then allot more has to much better ! this thinking is wrong in most cases !

if we can in any way get to the back side of a panel we will not use a weld primer .
the reason is simple it's just a waste of time and materials .

the problem with letting a epoxy or etch primer just burn off is the area around the burned off you will have flaking burnt primer . the little flakes that are half stuck to the back side of the panel you will find it real hard to get protection back on the panel under flaking burnt primer .
this will always leave you with a bare metal area for the life of the car . will that area blow out later on in life ? you know it will ! may take some years but it will always just keep rusting until it's rusted thru .
this is the type of thing a collision shop doesn't tend to care about but a resto shop does .
the reason is by the time the average car owners seam rusts out from not using weld thru primer the car no longer is owned by the person that had the repair done .

i have read some posting replies were the post reply stated allot of auto makers do not recommend using weld thru primers ! this is a faults statement !!!! there are many auto makers and as far as i know only 3 recommend not using weld thru primer .
if it was even half of the countless automakers saying not to use a weld primer it would still leave you with a hell of a lot saying it's ok to use it .
again it comes down to your knowledge of the product and it's proper use and when and why to use it more then just giving up on a product .

Last edited by 962porsche; 07-14-2017 at 03:28 AM..
Old 07-14-2017, 03:14 AM
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Thanks very much 962. I read the thread you referenced and your point in that thread about the zinc not needing to be everywhere was interesting (and surprising). So now I can see why the burn-off around weld thru is less of an issue than the same around epoxy.
Old 07-14-2017, 04:23 AM
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the misconception with zinc is you have to cover the bare metal to protect it . this is faults .
all you need to do is have zinc next to a bare metal area for it to protect the metal .
if your a boat owner and your boat is in salt water you will know the zinc plate you bolt to say your rudder helps protect the metals from rust and oxidation in the area around the zinc block bolted to the rudder .
Old 07-14-2017, 04:55 AM
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i can't find the write up i did but i did come across .
another posting with weld primer on page 2
question for 962 on welding flares is the title .

when it comes to weld primers there has been the debate for years .
some techs like using it others dont .
the write up I-car did about fiat and chry. is the same debate .

what it seams to always come down to is the knowledge of uses and when to use it and when not to .
one problem is people tend to use way to much weld primer . this can and will contaminate your weld . this is why some only want you to use weld primers with a resistance weld .
i have said this countless time many people think if a little is good then allot more has to much better ! this thinking is wrong in most cases !

if we can in any way get to the back side of a panel we will not use a weld primer .
the reason is simple it's just a waste of time and materials .

the problem with letting a epoxy or etch primer just burn off is the area around the burned off you will have flaking burnt primer . the little flakes that are half stuck to the back side of the panel you will find it real hard to get protection back on the panel under flaking burnt primer .
this will always leave you with a bare metal area for the life of the car . will that area blow out later on in life ? you know it will ! may take some years but it will always just keep rusting until it's rusted thru .
this is the type of thing a collision shop doesn't tend to care about but a resto shop does .
the reason is by the time the average car owners seam rusts out from not using weld thru primer the car no longer is owned by the person that had the repair done .

i have read some posting replies were the post reply stated allot of auto makers do not recommend using weld thru primers ! this is a faults statement !!!! there are many auto makers and as far as i know only 3 recommend not using weld thru primer .
if it was even half of the countless automakers saying not to use a weld primer it would still leave you with a hell of a lot saying it's ok to use it .
again it comes down to your knowledge of the product and it's proper use and when and why to use it more then just giving up on a product .
This is very sound advice. My point with the automakers is there seems to be a trend going away from weld thru versus towards it but I guess only time will tell. But you're right initially I thought there were more not using it.
Old 07-14-2017, 05:03 AM
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the misconception with zinc is you have to cover the bare metal to protect it . this is faults .
all you need to do is have zinc next to a bare metal area for it to protect the metal .
if your a boat owner and your boat is in salt water you will know the zinc plate you bolt to say your rudder helps protect the metals from rust and oxidation in the area around the zinc block bolted to the rudder .
Interesting and smart
Old 07-14-2017, 05:04 AM
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I picked-up some copper weld thru the other day (U Pol; my zinc one didn't spray out well at all). It just occurred to me, does the copper work the same as the zinc in the sense that it will protect metal that it is near (like 962's reference to the zinc plate on a boat rudder)?
Old 08-25-2017, 06:12 PM
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i hate the spray can weld thru primers at times it's a must to use them but most of the time i use the brushable weld primers .

zinc is a better rust protectant then copper is for steel when it comes to weld primer.
Old 08-26-2017, 06:03 AM
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