Dirty Welds Are Never Strong so Clean Your Metal Before Welding
The key to any good weld is clean metal, but what is the best way to clean metal before you start welding? Depending on the tools you have and the overall goal of the project, there are a few ways to prep your metal to get a nice clean weld every time.
The best welds come from pure clean metal-to-metal contact. Any foreign materials in the welding area can cause welding imperfections. Even brand new metal must be prepped before it can be welded because there is usually a coating put on new metal so it does not rust or oxidize during the shipping process. This is a factor that is often overlooked and will always result in a weak and ugly weld. Be mindful that once you remove this coating, the metal is exposed to the elements. If left out unprotected, steel will begin to rust — even indoors.
To start, the type of welding you are doing will determine how you prep the metal. MIG welding steel does not need the metal to be perfectly clean. On the other extreme, TIG welding aluminum requires contaminant-free metal to create a strong clean weld. In all the examples below, you can see the difference in the dull color of the “new metal” (left) compared to how it looks after it is properly prepped (right).
Angle Grinder with Flap Disc
Using an angle grinder with a flap disc works great to prep steel for MIG or TIG welding. Mild steel does not require the surface to be super clean to get a good weld, making the flap disc a quick-and-easy option to remove large contaminants. In the picture above, you can see the left side is brand new untouched steel. It may look clean, but it has a thin coating as stated earlier. Once you remove the coating with the flap disc, all it takes is a quick wipe-down with Low VOC Eastwood PRE Painting Prep or acetone and you will be able to make clean and effective welds. The flap disc grinding method works great for heavy welding on chassis parts. This area is always exposed to the elements, which will build up contamination over time. Take the time and clean the metal first; you’ll thank yourself later.
Be aware that a flap disc will remove a lot of material, so don’t use this technique on thin sheet metal as it may compromise the metal’s strength.
Sand Paper
Similar to using a grinder, the classic sandpaper method will work great for MIG and TIG welding steel or stainless. The downsides are that it can be time-consuming and does not always remove all the coatings. Like using a grinder, you must wipe the metal down with Low VOC PRE or acetone before welding.
In the picture above, I used 80-grit sandpaper, it worked well by removing the coating but also left deep scratches that may not look good. Therefore, this welding prep method is best for areas that will either be painted over or hidden out of sight. Additional finishing or polishing may also be needed.
Abrasive blasting
If the metal you will be welding is very rusty and is not suitable to be sanded or removed with a grinder, another option to prep the metal is abrasive blasting (also known as media blasting or sandblasting). After blasting, the metal may look clean, but it will still need to be wiped down with Low VOC PRE or acetone to remove any chemical contaminants.
It should be noted that the abrasive material can sometimes trap pieces of other metals that can cause the metal you are welding to rust or corrode. Never rely on a blaster to prep aluminum for welding; it is very sensitive to contaminants that can get trapped even after wiping it down.
Cleaning for Aluminum TIG Welding
When prepping aluminum, there is a slightly different process that you will need to be mindful of. Aluminum is very susceptible to contaminants. Therefore, the cleaning process must be done in reverse to produce clean welds.
First, you must wipe down the metal with Low VOC PRE or acetone. This will remove any oils or grease on the surface. The next step is to remove any oxides on the surface of the metal. To do this, use stainless steel wool or a stainless wire brush on the area to be welded. Make sure that the steel wool or wire brush is used exclusively for aluminum to avoid contaminants from other metals. Once these tools come in contact with mild steel, they can transfer steel bits into the aluminum, which will eventually create rust. Finally, wipe down the metal again with Low VOC PRE or acetone with a clean cloth or rag. From here, you are ready to weld.
Things To Avoid When Prepping Metal for Welding
Regardless of the cleaning method, there are some crucial things to avoid when preparing your piece. Here are the most common welding prep mistakes:
- Leaving deep grooves or marks. Not only can contaminants get buried in these grooves, but they also make the weld more difficult to control.
- Cutting off too much of the surface. This can severely affect the metal’s strength and shape, especially when you’re working with sheet metal and other thin metals.
- Welding when the metal is wet. MIG and TIG welding should only be done on metal that is completely dry.
- Forgetting to wipe down the metal before starting. Even if the metal looks clean to the naked eye, invisible impurities can still affect the weld quality.
- Improper handling of chemicals. Since both our PRE Painting Prep and acetone mentioned above are chemicals, they should be treated with care. Wear proper goggles, gloves and other protection when using them, then cover the solvents and wash up thoroughly when done.
It doesn’t matter what kind of welding you are doing — it’s always important to take the time to clean your metal before welding. Not only will your welds look amazing, but they will also be a lot stronger, which is always a bonus.
Check out the Eastwood Blog and Tech Archive for more how-tos, tips and tricks to help you with all your automotive projects. If you have a recommendation for future articles or have a project you want explained, don’t hesitate to leave a comment.
Hi, Yes abrasive blasting is always one of the best ways to clean the surface before welding. Just make sure you clean the part with acetone or a similar cleaner before welding.
Thanks for explaining how to clean metal before welding! I’ve just started to do this as a hobby and I didn’t know that this could make the bonds stronger. I’ll definitely make sure to sand before welding in the future. Would it be practical to get an abrasive blaster for my home workshop or should I stick to sandpaper?
Correct, exactly why I advise to COMPLETELY remove the coating before welding. We do not suggest to weld galvanized without removing the coating.
Matt – be very careful of the fumes created when welding galvanized steel. Extremely dangerous! – the zinc that is burned off becomes a powder and will make you very sick if you breathe it in. Always weld with proper ventilation.
Excellent points for the home or shop weldor!!! I worked at a National Laboratory Welding & Joining section doing high tech welding (laser, electron beam, automated GTAW, explosive, stir, DLF, etc.) and the cleaning procedure was always mandatory in all of our weldments especially on exotic metals! We even had a separate electro-chemistry section that would complete the last preparation cleaning step before we would perform the actual weld! Then, we would still be required to mechanically clean some metals (aluminum, beryllium, plutonium, etc.) within 10 minutes of the welding procedure! Clean metal is absolutely critical for a sound weld!
A very informative article. I have always been a proponent of properly cleaning and fitting the metal before welding. Takes a little time but how do you want to spend it, either by properly cleaning and producing a quality professional weld or spend just as much or more bugger welding poor fit and preparation, and an unqualified weld. Wouldn’t believe the people who can not or will not get this through their thick heads.
You would need to clean the entire galvanized coating off of the metal to get it back down to bare metal in the area that you’re welding. It spatters so much because you have to burn through the coating to get to bare metal and the coating creates a dirty weld which causes excessive spattering.
How to clean galvanized metal for Mig welding and why is it so difficult to weld without spattering ?
Great suggestions We have found the 3M open weave discs to be excellent They are in the Eastwood cataloge They are a in drill drive roloc and disc Work great last well and dont contaminate surface Advantages out weigh the cost Jeff
When I was much younger I had the misconception that welding would clean the metal while welding. Thanks for this very helpful article. I will now use pre also on the project before welding it.
Hi Bill,
I’d suggest going over the entire exposed frame with our cylindrical wire brush kit to knock off any loose, flakey rust. I’d then coat the chassis with our rust encapsulator. If you want a finish close to original you could apply our Rubberized Rust Encapsulator.
I have an 02 Chevyy 2500 HD that I am replacing the rockers and cab corners on. Since I have the box off what process and product do you recommend to clean and seal the RUSTY frame with? I think they may have put a rubber sealer on it from the factory , but I’m not sure. I live in salty Iowa. Thanks