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Q: I made a square tube by welding together four pieces of 2 1/2x3/16-inch flat bar. I put it together so the edges made a groove that I filled by MIG welding. Is it better to grind those welds down or leave them alone with regard to the strength of the weld?
J. P. Machado<>br />Via the Internet
A: Well, you certainly won't IMPROVE the strength of the joint by grinding the weld, so the grinding would only be done to make the joint look more finished. Appearance is often an important consideration for people who build classic trucks, so I want to be sure everyone understands how to treat a corner joint in a way that will yield the ultimate strength-on critical chassis components, a weld failure can cause a very unsafe situation, with possibly dire consequences!
The strongest way to make a corner joint is by using what I call a corner-to-corner fit, where the inner edges of both plates touch, and it leaves a V-groove as wide as the material thickness. If you make a full-penetration weld on a joint like this, grinding the weld flush will not reduce the strength at all-by your description, I think this is the approach you have taken.
I sometimes use a half-lap joint on an edge so that one plate overlaps half the thickness of the other. This weld is not quite as strong as the corner-to-corner joint, but it can be adequate in many situations, and it often makes the fit-up easier. Again, you won't reduce the strength of the joint very much by sanding the weld flat, as long as you have full penetration.
The least desirable way to configure a corner joint is to line up the edge of one plate with the face of the other. With this configuration, you will have a joint with considerably less strength than the base metal, and worse, you will drastically reduce the strength of the joint by sanding the weld. I've heard of many cases where people boxed frames on street rods and trucks using this joint configuration and then ground the welds flat, and it didn't take long before the welds cracked! It is possible to grind a V-groove in a joint like this, which will help you get better penetration, but I prefer the other joint configurations.
Q: I have always used a small brush dipped in acetone to clean welds on sheetmetal as a preparation for priming and painting. Is there something better? I am doing a patch on my truck just below the windshield, and I want to minimize the chances of rust reappearing. The area was stripped down to clean metal before welding. I am concerned about cleaning any contaminants that form during welding, like those dark-burnt smoky spots that form when tacking. I was under the impression that welding creates an environment on the metal that will promote rust if not cleaned properly after the welding is completed.
After I make a weld, I use acetone and a brush to clean the area. I wasn't sure if there was another process that might be better for final cleaning before applying primer and paint. By the way, this is the first time I have used Easy Grind wire with my MIG machine, and it is noticeably different-a little dirtier, but definitely faster to sand.
Marc
Via the Internet
A: As you probably know, rust is sort of like cancer-once it starts, you have to remove every trace of it, or it just continues to grow. The best time to clean the metal is BEFORE you start welding-as you have! I usually clean the metal before I even cut out the damaged area, since sometimes while cleaning, new holes or thin spots reveal themselves, requiring a larger hole to be cut for patching.
Acetone and a wire brush will remove any grease or oil, but it really isn't very effective on rust or paint. I normally use a 3M Clean & Strip disc to clean the metal, and it's important to get the INSIDE just as clean as the outside. Welding always causes some discoloration in the heat-affected zone, and this oxidation should be cleaned before priming. Sanding or using the Clean & Strip disc is an effective way to remove this discoloration.
After welding, it's important to get some primer on the patch quickly, both inside and outside, before the metal has a chance to start rusting again. I use a product called Metal Prep as the final chemical cleaner before priming. It will take off any traces of surface rust, and it leaves a phosphate coating on the metal to retard further rusting.
If you are getting a lot of spatter, you might want to check your machine settings-sometimes if you are welding with too much voltage, you will get an objectionable amount of spatter. Also, be sure to use an argon-CO2 gas mix for welding sheetmetal-MIG welding with straight CO2 causes a lot of spatter. And while I'm thinking about it, I want to remind all our readers that flux-core wire is not the proper material to use for bodywork!
Many people like using the Spoolarc Easy Grind wire-it is made by the ESAB Company, and any welding store should be able to order it.