# Plug welding with flux core wire welder



## Drewm (Jul 27, 2021)

To make a long story short, my car had a resto started 25+ years ago. Stopped for whatever reason. It looks like they stopped in the trunk. The drivers side of the trunk has been repaired. The passenger side was prepped for plug welds and then stopped and everything (the whole car) was painted/primed. Here you can vaguely see the seams (filled with seam sealer) and repair of the drivers side trunk floor. The wheel well and lower 1/4 panel is still in need of patches. 










The passenger side was prepped for a similar repair. Holes drilled in a patch panel and brace that is also a brace for a body mount:




















My welding skills are marginal at best, and I only have a flux core wire welder. Does anyone know if plug welds can be done with a flux core welder (Google has not turned up much)? Conversely, I am not opposed to having a body guy do this. In fact, I would prefer a body guy do this, but since it is in the trunk and will be covered with thick paint and some kind of trunk mat, I guess I am not too concerned about pretty welds. Also, would it make more sense to weld the body mount portion of the brace to the patch panel first, then worry about welding in the patch panel. Or, weld the patch panel in first, then weld the brace to the panel from underneath?


----------



## Sick467 (Oct 29, 2019)

I have done more than my fair share of body welding with a flux core unit and you can do it all with one. Make sure you keep any air movement (breezes) to a bare minimum. Any air movement significantly affects the weld quality and will cause more after-the-fact weld dressing. Flux core welders tend to produce more pin holes and, what I call, "booger welds", but they will produce good welds for sheet metal joining and plug welds. What you will find is that there is a lot more clean-up work to be done after the weld is laid. Plug welds are definitely the easiest to perform, however. What I found was the booger weld would be laid down and would often times build up above the panel and have to be ground back flush for aesthetic purposes. If the weld looked good before grinding, more times than not the grinding would uncover pinholes or hidden bubbles and have to be touched up with a second weld. This rarely happens with a gas bottle in play and I can't tell you how happy I am with the gas upgrade...so much less rework and better results. Just so you know, I replaced rear quarters, rear tail panel, multiple floor patches on a car all with a flux core along with cab corners, rocker panels, 11 foot of lower bed panels, lower door panels, and a few floor patches on a truck. These were my first sheet metal projects. I was a novice to say the least when I started them, but they turned out really well. Starting with plug welding is a great place to begin learning this art. Once you get the hang of them...move on to seam welds. They are just a long drawn out series of spot welds. What ever you do, don't just start welding at on end of the seam...move the spot welds around so that heat can dissipate and the sporadic spot welds will keep the panel from shifting on you . This goes for plug welds that are close to each other too. I always did a plug weld and let it cool entirely before performing the next in line, or moved several inches away a did another. It will help if you do the first plug weld in a series at the middle one and work your way out to the end of the line of holes. This way if the metal stretches from the heat, it can only stretch half as much before getting to the end of the line. Then work it the other direction. It' not that big of a deal, just a trick to help keep it as good has it can be.

As far as filling in a hole with flux core...You want as little contaminants as possible (none really). Rust, primer, paint, goo, and what-nots will make flux core welding more difficult than gas units. So, prep is even more important. I typically start the hole filling weld with the welder turned done low and "pepper" the edges until it starts to build up...like mini spot welds around the perimeter of the hole, then turn the welder up slightly and start making inward circles until it fills. The same works for plug welding where you have a solid piece under the hole, but is easier due to the extra amount of metal to take the heat. I would often use self drilling screws to hold the panels tight together (where clamps could not be used), close to the plug weld, and then go back and fill the screw holes. This sounds like a lot more work, but the results will be much better.

If you decide to go for it, practice on some old crappy sheet metal first, start on the car where it can be cut back out easy (in case it goes south), and post back with any questions...I'd be happy to lend my experiences.


----------



## armyadarkness (Dec 7, 2020)

Im a welder/ fabricator with my own shop, for the last 35 years, and Ive done many GTO's. Flux cored will work fine but the clean up will be a lot more involved


----------



## armyadarkness (Dec 7, 2020)

Sick covered it very well, in detail. Flux has it's place... I have $25k worth of welders, but whenever I pull out the flux, many gear heads mistakenly assume that I'm poor and can't afford a gas welder... The reality is, flux is much better at digging in and welding heavy stuff, and it's the ONLY mig which works in the wind.

If you're just doing patches, flux will be fine. You never see concours cars with patches anyway, so I assume you're not planning on selling this car at Barrett Jackson. 

Nearly all OEM welds were spot welds, which really suck, so flux will do the job well, but be sparing with it, because you and the 4" grinder are going to be spending so much time together that your wife will be jealous.


----------



## Drewm (Jul 27, 2021)

Thank you both for the advice.


----------



## Drewm (Jul 27, 2021)

Would you guys weld the brace to the patch panel, then weld the patch panel in, or would you weld the patch panel in and then weld the brace in from underneath?


----------



## armyadarkness (Dec 7, 2020)

I'm personally not seeing an advantage either way. Do whatever works best and is easiest. Welding upside down (inverted welding) is typically not for the faint of heart. There's a real science to it and when working with thin gauge sheet metal, you won't have a lot of time to learn before you burn through.

If your not skilled in it, avoid inverted welding wherever possible. I do.


----------



## Drewm (Jul 27, 2021)

armyadarkness said:


> I'm personally not seeing an advantage either way. Do whatever works best and is easiest. Welding upside down (inverted welding) is typically not for the faint of heart. There's a real science to it and when working with thin gauge sheet metal, you won't have a lot of time to learn before you burn through.
> 
> If your not skilled in it, avoid inverted welding wherever possible. I do.


My thoughts exactly regarding inverted welding, particularly with a flux core splattering down. I guess welding the brace to the panel first would minimize having to so much upside down. I appreciate the advice. Thanks


----------



## armyadarkness (Dec 7, 2020)

Im covered head to toe in burns from inverted... and I have an entire wardrobe of clothing with holes in it. If you can get it from up top with those drilled holes, then go for it. Otherwise...


----------



## NOS Only (Nov 14, 2017)

armyadarkness said:


> Im a welder/ fabricator with my own shop, for the last 35 years, and Ive done many GTO's. Flux cored will work fine but the clean up will be a lot more involved


100% correct

As someone who goes back to the days of brazing, you need to clean the weld area very well or filler will not adhere.

I have found that sandblasting the area is the only way to assure the area is totally free of contaminants.


----------



## armyadarkness (Dec 7, 2020)

NOS Only said:


> 100% correct
> 
> As someone who goes back to the days of brazing, you need to clean the weld area very well or filler will not adhere.
> 
> I have found that sandblasting the area is the only way to assure the area is totally free of contaminants.


I get at least 20 young guys, every year, wanting to learn to weld. As soon as I explain that welding is 15% layout, 40% cutting, 15% cleaning, 20% grinding, 8% painting, 2% welding, they're done wanting to learn!!!


----------



## Drewm (Jul 27, 2021)

So hitting the weld areas with something like a twisted wire brush in an angle grinder wont give a clean enough surface for an adequate weld?


----------



## NOS Only (Nov 14, 2017)

armyadarkness said:


> I get at least 20 young guys, every year, wanting to learn to weld. As soon as I explain that welding is 15% layout, 40% cutting, 15% cleaning, 20% grinding, 8% painting, 2% welding, they're done wanting to learn!!!


LOL!

Too much TV


----------



## NOS Only (Nov 14, 2017)

Drewm said:


> So hitting the weld areas with something like a twisted wire brush in an angle grinder wont give a clean enough surface for an adequate weld?


Nope


----------



## NOS Only (Nov 14, 2017)

Get yourself a cheap Harbor Fright handheld spot blaster. It'll do the job.


----------



## armyadarkness (Dec 7, 2020)

NOS Only said:


> LOL!
> 
> Too much TV


Exactly!


----------



## armyadarkness (Dec 7, 2020)

Sandpaper, a grinder, and a wire cup brush will all work, but as pointed out, the cleaner it is, the easier the welding will go. Sandblasted, clean metal welds like a dream... Rusted, oily, painted metal will all weld, but now you'll be struggling. Do a bench test... lay your painted and weathered parts out and weld a spot, then sand blast some scrap and try that, you'll fall in love.

If welding is ice cream, sandblasted metal is hot fudge, wet walnuts, and cool whip.

Nevertheless, do your best with what you have. But since that brace will become inaccessible after you start welding, get it bare and clean as possible!


----------



## Sick467 (Oct 29, 2019)

Don't stress the absoluter cleanliness of the metal. Super clean is better than real clean is better than just clean, but clean is key. Often times a sandpaper flapper wheel is all you need to do it quickly, but a regular old piece of sandpaper and some elbow grease will work fine. A sandblaster just does it really well and really fast (besides the sand everywhere).


----------



## NOS Only (Nov 14, 2017)

Sick467 said:


> Don't stress the absoluter cleanliness of the metal. Super clean is better than real clean is better than just clean, but clean is key. Often times a sandpaper flapper wheel is all you need to do it quickly, but a regular old piece of sandpaper and some elbow grease will work fine. A sandblaster just does it really well and really fast (besides the sand everywhere).





armyadarkness said:


> Sandpaper, a grinder, and a wire cup brush will all work, but as pointed out, the cleaner it is, the easier the welding will go. Sandblasted, clean metal welds like a dream... Rusted, oily, painted metal will all weld, but now you'll be struggling. Do a bench test... lay your painted and weathered parts out and weld a spot, then sand blast some scrap and try that, you'll fall in love.
> 
> If welding is ice cream, sandblasted metal is hot fudge, wet walnuts, and cool whip.
> 
> Nevertheless, do your best with what you have. But since that brace will become inaccessible after you start welding, get it bare and clean as possible!


Just to clarify......

My point of sandblasting the area clean was not for welding purposes, it was for body filler adhesion.

If you re-read my post I was referring to using the flux core wire and how the area must be clean afterwards for the filler to stick. I found that sandblasting removes any trace of flux, scale and contamination that would prohibit body filler from bonding to the metal.


----------



## PontiacJim (Dec 29, 2012)

Drewm said:


> So hitting the weld areas with something like a twisted wire brush in an angle grinder wont give a clean enough surface for an adequate weld?


With mig welding, the metal/welding works best when the metal is as clean as you can get it. It doesn't take well to rusty/scaly metal - that's where arc welding has the advantage.

As pointed out, you don't want any breeze blowing on the welding process as this can introduce oxygen and you get poor welds. So don't have a fan blowing on the weld area or a good breeze from outside blowing in.

You may also want to make sure that the polarity of the welder is set correctly for the flux core wire. I can't recall, but you can either have a +Positive ground or -Negative ground by switching the cable connections inside the mig welder where the power cable/ground attach. Same works for arc welding, some types of welding rod respond better with a +Ground while other respond better to a -Ground - and this also can be dependent on the metal composition.

The think most fail at is setting the heat high enough, you don't want too high a setting or you will burn through, but not high enough and you get crappy/lumpy welds that need a lot of grinding IF they stick. So you want to balance weld heat with wire feed speed. If the heat it too hot and wire speed to slow, you burn off the wire at the end of the welder. Too low a heat and too fast a wire speed, the wire stick out will get longer and longer like a tongue, then you have to cut it off and start again.

You can control heat by squeezing the trigger on/off, you don't have to keep the welder wide open or continuos. You can "stitch" weld, or do 1" at a time. If you do a continuous weld, you are more apt to warp the sheet metal from the heat. You are better to tack weld the sheet metal in several places to keep it from moving an warping. Then come in and fill those areas in between the welds. Jumping around gives the metal time to cool and will warp far less. It is much easier to stop if the welds look crappy and make adjustments rather than just keep on keeping at it hoping the welding will improve. If you have to grind out a bad weld, do so, don't try to weld over it.

Best thing is to practice on scrap that is like the metal you will weld. This will also help to dial in the heat/wire speed before doing actual welding. Mig is pretty simple to pick up, just have to get technique down and the machine adjusted and then it is just a matter of following the line of metal you want to weld together. Plenty of YouTube videos on Mig welding that are very helpful.


----------



## Drewm (Jul 27, 2021)

A few questions for the welding/body experts. I looked at the other side of the trunk, and it also has been patched. From what I can tell, he welded the brace to the patch panel, then welded the patch panel in, then probably did some spot welding from underneath the car. So I am going to weld the brace to the panel first, due to the fact that it will be much easier to do that on my workbench in a controlled environment. However I see no reason that the patch panel needs to be that large. Underneath looks really good, other than the area that was cut out.

So...

1) Do I cut the panel to the size that I want before welding the brace to the patch panel?
2) Do you see a reason to keep the panel that large? I dont want to cut out good metal if it is not needed.
3) If you look at the close up picture of the trunk (the first picture), you can see where the metal was recessed, I suppose to make the plug weld once the panel was ready to be welded. Is there a tool that recesses that lip in the metal, or is it just a matter of a hammer and some knowledge of how to work the metal? Maybe the previous guy was also thinking about trimming up the panel first?

Since this is in the trunk, and will be covered by heavy paint and a trunk mat, I am not too concerned about the weld being pretty. I just want it to be strong and correct.

Thanks in advance for the help.


----------



## PontiacJim (Dec 29, 2012)

Drewm said:


> A few questions for the welding/body experts. I looked at the other side of the trunk, and it also has been patched. From what I can tell, he welded the brace to the patch panel, then welded the patch panel in, then probably did some spot welding from underneath the car. So I am going to weld the brace to the panel first, due to the fact that it will be much easier to do that on my workbench in a controlled environment. However I see no reason that the patch panel needs to be that large. Underneath looks really good, other than the area that was cut out.
> 
> So...
> 
> ...


The patch panel may have just been that size when ordered. I see it eliminates the oval plug/plate in the original floor. It appears to already have a "lip" on the left edge which would go under the existing floor. They sell replacement floors as 1 piece or 3-piece depending on how much needs to be replaced or if you are removing the body off the frame to replace the trunk floor in 1 piece.

You can do it any way you want. I would bolt the brace in place first so you know it is on correctly. You don't want to weld it to the patch panel and then find that it is off and won't bolt down.

Trim your replacement panel however you choose to use it. Then lay it over the floor and outline it with a Sharpie. Then cut the floor along the Sharpie line. A high speed die grinder with a cut-off wheel is a good tool for this job. The trim as needed to fit the replacement panel within the hole you cut out in the floor. The two panels should "butt" together. You will then weld up the 2 pieces, like a seam, all around the patch. Tack weld all around the patch to make it secure. Keep the heat going into the 2 pieces down, so not long hot welds. Then go back and make small stitch welds to fill in between the tacks, making sure you weld opposite from where you just did a stitch weld to keep heat down, continue to jump around like that until it is all welded up.

If you did not bolt up the brace, bolt it up and fit it where it should go. You then want to draw the floor down tight onto the brace. I might drill several 1/16" holes, or something small like that, through the floor and brace flange. Then insert a bolt/nut and tighten down to draw the 2 together. This will hold it into place and clamp it tight to the floor. You will weld up these holes when done - just like a plug weld.

The holes used for a plug weld will be larger on the side you are welding on. You have to get the weld through to the brace flange. Those holes I am seeing now look a bit too small and the minute you go to weld, you may only close up the hole and not penetrate down to the brace flange. It may be the photo? Does not really matter the size of the hole as you will be filling it in with weld and then grinding flat. Again, jump around when spot welding so as not to get the pieces too hot and warp. You can do a couple welds and walk away to let it cool and then come back, there is not rush. If you get any warping, then you can use a flat body hammer/dolly to beat it flat - but it will most likely take 2 people seeing where it is located, one person on top with the hammer, one underneath with a dolly.

That is how it could be done, but you will have to figure out most of this seeing you are doing it. Just measure twice, cut once, and take your time welding it up. Yep, it will get covered up with a trunk mat, but you want to have it the best you can because in time, it will bug you that it was not a good job and eventually you will be back at it fixing it "better."


----------



## 67ventwindow (Mar 3, 2020)

I am not a sheet metal guy. I was taught to weld on farm and heavy equipment. But this guy has some interesting twists on repairs.


----------



## mianhbht (Aug 10, 2021)

PontiacJim said:


> With mig welding, the metal/welding works best when the metal is as clean as you can get it. It doesn't take well to rusty/scaly metal - that's where arc welding has the advantage.
> 
> As pointed out, you don't want any breeze blowing on the welding process as this can introduce oxygen and you get poor welds. So don't have a fan blowing on the weld area or a good breeze from outside blowing in.
> 
> ...


Exactly.


----------



## Jd70 (Jul 4, 2019)

Check out this guys instruction video on just about exactly your issue.






He’s from Nova Scotia so has a bit of accent. He only uses simple tools and has a real knack for instruction, and he *doesn't* waste your time.

JD


----------



## Drewm (Jul 27, 2021)

Heres another video by the same guy that made me feel a whole lot better about my welding ability (or lack thereof) with a flux core...


----------



## Jd70 (Jul 4, 2019)

Thats one of his longer ones but sure splained alot

JD


----------

