sheet metal welding question

Hi to all,
My grinding skills are improving, but my welding leaves a lot to be desired. I am working on the floor pan at this point in time. Rockers are next. I understand that I need to cut out the rust and then prepare a patch to fit the hole. I am trying to but-weld and not have an overlap with spot welds. I first tack the patch with just a touch of heat in several places, then attempt to fill in around the patch without over-heating the metal. So far, by the book (I think).

The problems that I have are these: (1) I end up with little mounds of weld that takes a lot of grinding to make smooth, and (2) there are a lot of holes in the weld when I shine a light upwards from underneath. This is from the tack welds not running together as I fill in.

I have discovered that the rust has also thinned the remaining sheet metal to the point where I get serious blow-thru even on the lowest voltage range (Lincoln Electric MIG 140 using .025 wire and 75/25 C02 / Argon). I have somewhat solved this problem by using the lowest voltage range and a slower wire speed.

So, should I work on developing a better weld bead? Or, go to an overlap with rosettes? I know that this is the floor, and that I can cover up most of the unsightly work with fiberglass. But, I eventually will need to do a good seam weld with excellent strength with the outer rocker / sill / doorpost / longitudinal work.

This restoration is for a daily driver, not a show car. But, I want to learn good techniques and do it right.

Thanks in advance,
Rick

Use a copper block underneath the area and press it up against the area you're welding on the opposite side. This will keep the area your welding cool enough to prevent blow thru. I would also skip weld every couple of inches to help prevent the sheet metal from warping and then go back and fill in the empty sections. Low heat, move fast. You'll need a partner. Hope this helps...

Sounds like you have something contaminating your weld. Have you thoroughly cleaned the other side of the metal? If there is paint, undercoat, dirt, etc it can cause this problem. Prep work is 90% of a good quality weld. It has to be cleanly shiny metal.

Also, have you checked your gas settings? Is your weld clean or does it seem "dirty" or "sooty"?

You can also use copper (I use flattened copper pipe) to hold underneath a blow through when you need to fill the hole as the weld will not stick to it.

To prevent warpage you can also use a blower tip if you have access to compressed air. Search on youtube for stitch welding. I make about 4 stitches and the blow cool with compressed air. I use .6mm wire, not sure what .025" is in mm?

Practice on some scrap! Play with your settings until you are happy.

I am also an inexperienced welder - especially on thin stuff - and have been chasing a similar dragon. I fabricated and was installing rear swaybar mounts when I discovered that the bottoms of my rear seats were quite "crusty". Upon further exploration I also discovered that the right side of the parcel shelf was pretty shot.

When trying to patch the seat bottoms I had the exact same problems you are having and tried most of the same solutions. Everything seemed to help a little bit but what seemed to help the most was a change of technique. I found that getting heat into the area somewhat gradually allowed me to follow-up with a short bead without blowing through. What seemed to work for me was a couple of VERY quick pulses (as fast as I can cycle the trigger) into an area of solid metal and then immediately back on the trigger to draw a short bead. Aiming the gun into the most solid metal seems to help too. And like others have mentioned, I worked around the piece in sections so as not to get too much heat in one area.

I can't explain why it worked for me but it almost seems that getting that little bit of heat into the material first helps get the heat moving so it doesn't concentrate in one spot. The small amount of weld deposited by the quick pulses may act as a kind of heat sink. Regardless it's been fairly consistent and I've had better results with that technique without a copper backing than simply getting on the gun with copper.

Aside from that, fit of the patch makes a huge difference as do machine settings.

Good luck!
Stuart

Rick,

Join this forum, and post some photos of your welds..... They'll sort it out.

If you're ending up with large mounds in your plugs, it sounds like you need to dial the wire speed back.

As suggested get some scrap and practice.

Start by trying to plug weld a bigger hole, and work down to what you're doing on the car.....welding just takes practice, like most things.

Hope that helps

thanks to all for the advice! Rick

Rick, there are a few issues that you've identified in the explanation of your welding problems. First, cut out ALL the rust, do not leave any rusty metal, it is compromised as per your observation of pinholes. Rust does not weld properly. It will not stay welded and after all your hard work, why would you not do a complete job anyways? "...I have discovered that the rust has also thinned the remaining sheet metal to the point where I get serious blow-thru even on the lowest voltage range (Lincoln Electric MIG 140 using .025 wire and 75/25 C02 / Argon). I have somewhat solved this problem by using the lowest voltage range and a slower wire speed..." A recipe for disaster...cold welds on rusty metal. Yes, you might wind up replacing the entire floor, but at least the car will be safe. Weld metal must be clean unless you are using a stick welder and stick welding sheet is nearly impossible. Clean paint and undercoat off the pan and rocker with a flapdisk on your grinder and then clean with acetone before hitting it with the wire. Gas weld wherever possible clean, shiny metal, try not to use flux-core wire, completely cleaning flux out of a weld is difficult and it is easy to get stuck in a weld (inclusions), paint doesn't like to stick to the flux either, often causing problems with adhesion, discolouration and chipping....how much is a decent paint job nowadays?

When you plan to cut out various sections of floor, rocker, etc. make sure that you only work one section at a time to ensure structural integrity and "straightness". Removing all the metal that you plan to replace at the same time can be a recipe for disaster, especially in a Targa or Cabrio. Removing floor and rocker sections will allow the car to fold, and in a coupe you can easily wind up with misaligned panels, so one section at a time, unless you have a Celette fixture to ensure straightness. Welding in your floor pan is a relatively easy task (pay attention to fit-up), but it becomes more difficult the less welding practise that you have (hood time), in other words practise first and practise often. Get some steel that is the same thickness as the pan/rocker, and practise on the scrap, not the car. I believe that it is 20ga, possibly 22ga... the floors might actually be 18ga, I recall reading something along that line, perhaps a little research is in order. Be careful to ensure that you get full penetration on your clean welds, cold weld can create a disaster. In MIG welding, make sure that your electrode travel speed is good. Running a little hotter with a CONSISTENT speed will ensure decent results. A copper or Alu backing plate (+1/4") is also a very good idea as it will dissipate most of the excess heat in the weld and help minimize blow-through. Again, practise before you weld so that you can make all your mistakes on your bench, not on your work. Do not attempt to make a continuos weld on sheet metal, too much heat develops in the sheet and it will blow-through. Develop your stitch welding technique: make welds 1"-2" in length about a foot or so apart, moving around the pan /rocker, the welds will eventually start connecting together. This technique allows the area you just welded to cool and minimizes distortion. All your short welds will eventually connect together to make one continuous bead, just make sure that everything wets in when the stitches come together. There is good info online detailing the technique, it's quick and easy to learn and an absolute necessity for bodywork. What are the 3 rules of welding? Practice, practice and well, practice.

Your newly gained welding expertise will be for naught if you don't learn the MOST important part of replacing steel, which is FIT-UP. Making sure that there is a consistent gap between original metal and new donor sheet will ensure that you make the best welds...no large gaps next to overlapping steel. Take the time to ensure all new steel is properly fit for the strongest welds, it will make every step along the way easy breezey rather than compounding issues into a nightmare and winding up with your seat (and your butt) scraping along the road at 100 mph because cold, improperly fit welds broke. Might be great way to deal with a passenger if you're looking forward to an imminent divorce, but not for your daily driver. ;)

Sounds like you are not too far off getting good results, the more questions that you ask, the better. Online resources are incredible these days, deep research is readily available at your fingertips as well as plenty of helpful advice from friendly and knowledgeable people on fora like this.

Once you get things down you will really be able to read your welds, they will tell you everything you need to know about wire speed, travel speed and voltage, hot or cold. Enjoy your build, cheers!!

:D