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Hi folks,

I'm trying to repair a few small rust holes in the floor pan of a car. What started out as 1/8" before wire brushing and was 3/16" after cleaning is now 1" in diameter after my failed attempts to weld it. I was using a flux core 110V mig set at it lowest temperatures but it just keeps blasting through the metal even with a momentary burst of trigger. I really don't want to have to cut this floor back to strong metal (not sure how far that would be.....maybe have to replace the whole floor pan) and that much effort isn't worth the time. I've tried doubling up the floor pan with a piece of new metal underneath but no luck. How do I get this job done without going through heroics? Abandon the mig and gas weld it? Abandon the mig and braze it? Just spray the holes full of undercoat and pretend that they're not there?

Any suggestions on how to shortcut this job are very welcome.

Bill H.

If it is a non-strucural part of the floor, then just about any method should work.

1) Cut out a piece of steel 1/2" larger than the hole and adhesive-bond it to the cleaned existing floor.
2) Weld a new piece using butt-weld techniques. (difficult with a flux core wire)
3) weld in a new piece the exact size of the hole using a piece of copper behind the joint being welded.
4) same as 3 except try aluminum backing.
5) same as 3 or 4 except the repair piece is ~ 1/2" overlapping the hole. An offset flange would be nice if you can do it.
6) same as 5 except use sheet metal screws
7) same as 5 except solder it in place. Use muriatic acid (or "the Works" thick formula cleaner) to clean the metal and pre-wet the overlap areas. Flush clean with water and neutralize with baking soda after soldering.
8 ) a piece of metal flashing or cardboard and a whole can of roofing tar; top and bottom. (just kidding)

If it is a structural area, then one of the welding operations is the only choice IMHO

I'm sure there are other methods....

Hi Chuck,

Thanks for the list. Tell me more about #3. What does the copper plate do? Does it get welded onto the back of the repair? (It feels like some old plumbing pipe is about to get cut up!!)

Thanks, Bill

The copper needs to sit flat against the back of both sheets. It is used during the welding process to prevent the MIG wire from poking thru the joint and also keeps the molten metal from dripping away from your joint. The copper is not left in the weld as it will not adhere to the steel. It only need to be large enough to back 1 eld pass. I have a 1-1/4" x 1/4" thick copper bar about 9 inches long that I use for this welding method. Although I have never seen this mentioned, I have recently tried using aluminum in this same manner. It worked OK in the weld area but the splatters more easily melt into the aluminum than it does into the copper thus making the aluminum more difficult for the next use. Aluminum will melt at 1220 deg F while copper melts at 1983 deg F; over 700 degrees hotter. So if you have a block of aluminum and no copper, you might try that, especially with the tack and repeat method mentioned later in this post.

Another trick I have learned in thin metals is to angle (~45 deg) the MIG nozzle across the gap rather than line it up with the gap. If the speed of the feed is too high or your hand is not steady, the wire will hit the metal edge rather than directly shoot thru the joint. The more the noxxle angle, the more metal it has to burn thru. A 45 deg angle is effectively 41% more metal thickness to burn thru before you burn through.

Another method I did not think of is, you just tack-weld the repair piece in 3-4 places. Then come back and tack weld again next to the 1st welds, then again and again until you have a complete weld. This is a common method to limit panel warping but also works well in thin metal applications.

you need to cut it back to, as you put it, good metal, thats why it's called good metal!

do the job right or don't do it at all!

98 VW Golf. Holes rusted thru both rear passenger floorboards after enduring a Wisconsin winter w/ 125" of snow ("Yeah, we never get this much snow, really!").

Bought some tin that looked thick enough at Lowe's. Cut to shape w/ a tin snip. Pop riveted it in, using some washers to help support the rivets.

Used that expanding foam spray stuff and sprayed the everliving @#@$@#$!@!!! out of it. Got it on my hands. Had to do a job interview a week later w/ that stuff all over my hands, 'cause it didn't come off until the skin cells died. By that time it was black, which matched the car's paint, not to mention PORS.

Been that way for 3 years now, just passed the State of North Carolina's safety inspection (after I replaced that illegal dipstick tube and the criminal window tinting).

Done right!

Hi John,

I fiddled around with a variety of different attempts to do this job quick and easy and none of them worked, not even the copper bar trick, so I ended up cutting back to good metal and doing the job right . Still found that welding the patches in with a flux core mig was a challenge but worked at it until I found a method that was successful. As you can imagine that entailed real short bursts...bzt, bzt, bzt, bzt.....as I moved around the patch from side to side to side.

It was 34 degrees C here today (that is 93 degrees F for those of you that aren't ambidextrous!!), which is pretty warm for Canucks, so I was wearing shorts in the garage as I was welding. As I was kneeling on the concrete, leaning in the door to weld, I put some cardboard down to pad my knees. Well, you can guess what happened as I concentrated on welding the patch. Luckily I smelled the fire before it got to me, didn't even lose any hair off my legs. Note to self...find something fireproof to kneel on when its hot out.

Bill

Hey bill,

if you use a woolen rug or a blanket wetted down with some gas you could have some real fun!

don't ask how i know.

that aside, be careful about sun tan from the weldin in shorts.

Brings back childhood memories. I lived in a small Indiana town where a local eccentric was a nudist. And, he also made a living out of high altitude (church steeples, radio towers, etc) work. See where this is going?

Well, one day I (at age 16) was doing some work for him and his parents (he lived w/ his elderly parents) and damned if he didn't start doing some arc welding while wearing just a Speedo and old sneakers! I don't know how he did it, I think that most of us wouldn't have stuck around in that shower of sparks unless completely covered.

AT that point, he had my respect.

What size wire were you using for your gauge of metal, .035 for example will still require more amperage than .023.

Al

Hi Al - I'm using .030 flux core....didn't know that smaller existed, where do you get it?

Also learned that I adjust my wire feed way down and it 1) contributes to automatically doing the right size "spot" weld that doesn't burn through the thin metal and 2) cuts down on having to continually trim extra stick out when I do't get a good connection and no arc...its much easier to pull the trigger and get more stick out than it is to cut off excess all the time. I've also learned to gauge the color of the cooling weld and am now able to keep working in one straight line (after tacking all four corners of the patch) rather than have to jump around the patch all the time.....this really speeds up the process. Only real problem that I've run into was an area of the floor that had previously repaired with brazing, which won't weld, so I had to bring my brazing kit home from the lake today and I'll tackle fixing that tomorrow.

John - I dragged out an old leather jacket and put that over the cardboard padding. I don't think that I'll soak it in gas

Also, do not feel you need to continuous weld the pan as you would a fuel tank.

FYI, without sealer, production cars are not water tight. Thats what they make seam sealer for. You can buy it in caulk tubes from Mcmaster, Eastwood, or your local auto paint store.

One way to apply it is to use a wide paint brush on top after applying a bead to the area, to help push it into the seam. You can also temporarily add tape on the back side to prevent the sealer from flowing out when initially applied, waiting to cure.

Fully paint area BEFORE sealing. Cured sealer can also be topcoated.

I have Sil-thane 803 from Mcmaster.

I was arc welding a trailer hitch and had long pants, long sleeve shirt, mask, gloves etc. I was covered [almost] ........ I forgot that when I squatted down my ankles were exposed.

You guessed it .... the next day I could hardly walk from the sunburn!

yeh, i thought i was fully covered too but i had an old shirt on with a hole in the front and got this great big red blister from welding.

Dude
If you have a torch and can gas weld do it. Start with CLEAN joints. No messy flux or clean up. Just tack it well. Using the copper plate will help to.