View Full Version : Flux core ok for auto patch panels?

08-02-2009, 11:02 PM
Anyone use flux core for patching rotted sections on cars? Also, what gauge metal would be best for a 65 Corvair and 67 Cutlass? I'm using a 80amp rated Sears welder..Shoestring budget.

Thanks to anyone who can help guide this rookie.

08-02-2009, 11:16 PM
If you can weld in a patch panel with FCAW, you are good - and patient. FC runs too hot to be used easily for body panels. You'll burn through quickly, and that will get old quick. If you are able to weld in small enough spots to keep from burning through, the chipping of slag between welds will get old even quicker.


08-02-2009, 11:27 PM
My welder can be set up for gas..I was hoping to get away easy..What gas should be used for this type of project? Also, what wire?

08-03-2009, 07:12 AM
Hey Techmaven,
I do restorations on 4-6 classic/show cars a year that includes floorpans, kickpads, wheelwells, trunk panels, etc., & all restoration welding is with .023 wire & C25 Ar/CO2. Auto sheetmetal is quite thin(usually 22-24ga) although some of the older classics('60's) were 18-20ga & much easier to work with. Dave(whateg0) is quite correct with his summation to your query. Lose the flux-core & get some .023 & C25 & your success will be attained by doing practice on some scrap pieces the same thickness as you will be using. The metal has to be clean & bare to get a good weld & any rust will be detrimental to your efforts.

Once you get your wire/gas setup configured & some practice on scrap, your technique will determine ease or difficulty. You must maintain awareness that you will not be able to run continuous beads with auto sheetmetal.... tacking only. I also use varying lengths of 1 1/2" aluminum angle as a backing to absorb heat & provide a solid platform for the weld to adhere & reduce burnthru. It takes PRACTICE!!! Also, you will have better results by tilting your MIG gun to a sharper angle & this will allow your puddle to be flatter. You have to TAKE YOUR TIME!....hurrying will not produce optimum results.

Lastly, being on a shoestring budget will not enhance your success because you limit your variables and investment for a long-lasting repair. What good is a repair if it only lasts a couple months? If your goal is reselling, then go ahead & do the shoestring budget, but don't look forward to incoming recommendations as to the quality of your work. If this is for yourself & you plan to keep the vehicles, do a first-class repair & you will won't regret your xtra efforts. Also, if you maintain the namesake(Techmaven), your membership "handle", just do some homework & search the vast amount of information regarding auto metal & restorations to enhance your knowledge level for this kind of repair. Anyway, just MHO & experience with the classic/show car restorations I have done. Good luck.... Denny

08-03-2009, 07:58 AM
I'll just add...don not "Over-Prep" your already thin sheetmetal. Use aflap disc to get it clean, and stop. Doing more will give you something about as thick as a beer can, and you'll never get that to weld with your setup (after you go to MIG).

What will help when starting out is to add another layer of cleaned sheet metal of the same (but even thicker is better!) behind the seam...as a "backer" piece...this will just about eliminate blow-through, and "get your chops" up on the delicate spotting, gun angle, wire speed techniques. Not to be used where salt/dirt can accumulate under that backing piece, but I've used it on anumber of upper body parts that wee backed to the inside of the cab.

Another concern is warping and "oil canning" of body panels that comes from over-heating, causing portions of the patch panel and the "parent" panel to expand and stretch. This is controlled by keeping those little tacks as modest as possible, and be sure to spread them out...a lot I do the panel corners, then rotate to the panel centers, then 1/2 way between, etc.. I keep a Windex bottle filled with water to 'spitz' my welds to keep the heat spread under control.

Actually, the current panel adhesives (like Lord) are so good that backing strips can be fitted, then C-Clamped into place, then the new panel set onto the strips, that over-lap the seams...easy finish with body filler. Its what you have to use on the new high carbon sheet metal panels nowadays.

Positioning your perfectly matched panel (those Flap Discs sure come in handy achieving that fit!), and keeping a nice 1/16" seam, I use a lot of these Panel Clamps, which Harbor Freight has CHEAP...I bought my first set from Eastwood for about 4 x what I subsequently paid at HF. Appear to be the same product to me. (I don't find those listed on HF's website, but there just might be some in the stores. Seems like they are about $6.00 a bag of ten).:)


Like this...(just a down and dirty quicky in Photoshop...'could be cleaned up, but I think it conveys what I was attempting)

08-03-2009, 12:34 PM
I have used gasless flux core to do 20 gauge floors up to 24 gauge quarter panels. All butt joints no lap. Single spot weld, move etc. till panel welded in.

I have the the .024 wire and c25 also, it is higher quality, but when just doing spots the flux core was acceptable if set up properly. I did fight pin holes more with the flux core. Even small stitches it was no contest for the C25 though. In my opinion flux core limited to spots only on thin sheet.

Either method I could not eliminate buckling distortion no matter how careful I was, or how much I cooled. The next large thin panel I work on will be done using bonding agent.

Make sure you are getting proper penetration which ever mode you choose. When doing the spots its important to put dolly behind and peen them to relieve stress.

08-03-2009, 02:06 PM
I would think the bigger advantage to dollying the weld would be to stretch the metal back out - think metalfinishing. Do it well enough and no body filler needed.


08-03-2009, 02:09 PM
i just welded some new feners up on a trailer and the backing for the fenders were all 16guage.. i was using 030 fluxcore (until it ran out then finished with 035) it worked but since its 65 a Corvair and 67 Cutlass you are working on id try to get gas and 023 or 024.. probably tomorrow im going to be putting in a new floor, rocker panel, and cab corners on my 76 stepside. i think im going to try to at least tack them in place until i get a gas setup, right now i just cant afford the extra cash for the bottle. ill post how it works. ive heard before using a backing plate will help the burn through but have yet to try it.. i think i was told a copper backing plate but that could be wrong.

08-03-2009, 03:31 PM
Copper works great. I buy a copper pipe union at the hardware store, put a piece of welding rod or coat hanger in it bent into an "L", then smack it flt with a hammer, and finish it off in a vise. The rod gives you a nice handle. Now, this is different than a "double-thick" backing like I discuss above, which becomes part of the weld. Your weld will not stick to the copper, plus it acts as a heat sink to draw heat away from that spot on the panel.
Just go do it!:D

08-05-2009, 02:20 PM
I would think the bigger advantage to dollying the weld would be to stretch the metal back out - think metalfinishing. Do it well enough and no body filler needed.


You thought wrong. The more stretching you do the more shrinking thats needed. Peening welds relaxes the stresses helping to reduce distortion.

08-05-2009, 03:54 PM
No, I stand by what I stated. After you weld the seam, the metal contracts. Thus it must be stretched back out to be flat. I've never seen a weld on otherwise flat metal that needed to be shrunk more..


Charles Sand
08-12-2009, 07:59 PM
whateg0 is correct.

Old Sporty
08-13-2009, 06:26 AM
Bobj and Whatego are both correct. Peening the weld helps relax the stresses induced by the heat and electrical arc. Also when done properly, peening stretches the weld area out, reducing some of the shrinking when the weld cools.

08-13-2009, 07:31 AM
I do have a good VHS how-to tape on Hammer Welding. Has a lot of HTP promo interwoven, as I recall. I haven't looked at it in five years or so. Anyone want to give me the $20.00 (Shipping Included) I laid out for it, its yours. PM me. :)

BC Iron works
08-13-2009, 07:27 PM
i did my floor with a weld pac100
75/25 gas and 0.25 wire

08-14-2009, 06:53 AM
I've filled holes in 24 ga auto bonnet with .030 flux core wire. Can't run even short bead. Tack weld and hold position until it starts fading from vision threw #10 welding lens then another tack next to last slowly makes crude short bead. Much easier with .023 wire and shielding gas but your tempted to make longer welds with increased distortion. Any welded repair will have shorter life than original unless you can paint back side.