How To Prep Metal For Welding

When you work in manufacturing or metal fabrication, welding is critical for project completion. While most metals can be welded when you use the right technology for the material, it’s equally important to properly prepare the surface by removing dust, paints, oils, greases, and rust (oxides).

 

At Red Label Abrasives we have provided leading metal workers and fabricators with the abrasives they need for successful results, so in this blog, we detail how to prep metal for welding. 

Common Metal Welding Techniques

Gas Tungsten Arc Welding

The gas tungsten arc welding process, also known as TIG welding, uses non-consumable tungsten electrodes that generate an arc to provide heat. Since this method has a relatively slow speed, it is usually not used on carbon steels, although its clean results make it a common option for stainless steel and aluminum. 

Gas Metal Arc Welding

MIG welding, also known as gas metal arc welding (GMAW), uses a continuous solid wire electrode. Electrically charging the contact tip melts the wire and creates a weld pool between the two components. The spatter created during welding makes MIG best suited to projects where weld appearance is not important. Because MIG welding is faster than TIG welding, it is preferred for carbon steel.

Shielded Metal Arc Welding

Shielded metal arc welding (SMAW), which is also known as stick welding, uses a consumable electrode with a metal rod at its core. Heat is produced by the arc formed between the electrode and the base metal. Compared to other methods, such as TIG welding, stick welding is more cost-effective. Because of its portability and versatility, it can be used in any position and with any sheet metal thickness.

With shielded metal arc welding, you can usually get away with some surface impurities, but the process requires more interpass and post-weld cleaning. Gas tungsten arc welding and gas metal arc welding require a cleaner surface to produce quality welds but require less cleanup afterward.

Why is Surface Preparation So Important?

Steel needs to be clean of rust, acidic chemicals, oil-based coatings, and impurities like mill scale before undergoing heat-based processes. If it is dirty or has surface defects, it can affect the quality of the finished product by interfering with the welding, creating resistance, and even producing a weld splash. The strongest and most attractive welds are made on bare metal that is free of flaws or impurities. You want a pure metal-to-metal connection when welding.

Pro Tip: With some metals, especially nonferrous ones like aluminum and titanium, the window of time between surface prep and welding is comparatively brief, as oxidation can happen fast. If you clean large areas (e.g. sheet metal) long before you’re ready to start welding, you may end up having to rework it.

Best Ways to Prep Metal for Welding

Here are eight commonly-used ways to prepare metal for welding. You should keep in mind that certain tools will be more appropriate in certain situations than in others.

Cloth and Solvent

Weld defects can occur if the metal is impure or contains chemicals, especially with processes like TIG welding. You can often address these issues with a cloth and solvent, but make sure that you choose the right cleaner, as some of them can leave an oily residue. For a residue-free clean, use acetone or lacquer thinners.

Pro Tip: Be careful when using any flammable liquid on metal surfaces. If you operate your welder around flammable liquids, you could cause a fire.

Wire Brush

Metal workpieces can be cleaned with a wire brush to remove thick layers of mill scale, slag, or any other thick impurities. If you're stick welding, you'll need a wire brush to brush off the flux after you're done. You will need specific brushes for certain metals. Aluminum, for example, requires a steel brush, while carbon steel brushes are the best option for iron. The versatility of brass wire brushes allows them to be used on a wide range of projects.

While power brushes do a faster job, hand brushes are the better choice when working in tight areas. V-groove scratch brushes, which have rows of wire angled into the brush face, allow you to focus and reach those tight areas.

Pro Tips: 

• If you are using a stainless power brush to clean welds, run it at a very slow speed: between 1,250 and 4,250 surface feet per minute (SFPM).
• Avoid cross-contaminating your brushes with different metals. Use separate brushes for each metal to avoid other issues or use something you can throw away after each use.

Sandpaper

Prior to welding, metal can be sanded to remove impurities and imperfections, especially rust and old paint. You should, however, make sure you use the right type of sandpaper for the metal and welding application to avoid damaging your workpiece: for rust and paint, 80 to 110-grit should do the job. Any coarser and you risk creating scratches on the metal. 

Pro Tip: Sanding sheets, belts, and discs are offered with various abrasive grains. You can use aluminum oxide, but zirconia or ceramic will get the job done faster when sanding metal. 

Angle Grinder or Die Grinders

Angle and die grinders are among the most versatile tools you can own in a welding shop. For metal prep work, welders typically use angle grinders measuring 4-4.5”, which do an excellent job of cutting small metal pieces, cleaning metal, and smoothing rough edges. (There are larger angle grinders available, but they are more useful for applications other than welding.) After removing any coating from your metal with an angle grinder, do a final cleaning using acetone.

For weld preparation, angle grinders and die grinders can be used with the following tools/attachments.

• Wire Wheels and Brushes: This includes nylon wire wheels, end brushes, cup brushes, and stringer bead wire wheels. When working around uneven surfaces or bolts, they are a better option than sanding discs or grinding wheels.
• Strip Discs: Nonwoven sanding discs have abrasive grains that are aggressive enough to remove metal impurities without damaging the metal.
• Flap Discs: With flap discs, almost any material can be removed quickly, leaving a clean, flat surface for welding. However, they can have an aggressive cut, so be careful not to remove too much underlying material. Flap discs with a top-sizing in the coated material provide extremely aggressive stock removal rates while minimizing heat buildup on poor heat-conducting materials such as stainless steel.
• Grinding Wheels: With these wheels, the most material can be removed and the edges of a metal surface can be contoured more efficiently, but you run the risk of removing too much material.

The angle should be lowered when using a grinding wheel, as they are most aggressive on the outside corner, so the steeper the angle of approach, the greater the potential for the wheel to remove too much surface material and undercut or gouge the workpiece. Lowering the angle increases the wheel's surface contact with the workpiece and reduces its aggressiveness.

Pay careful attention to the pressure you use while grinding. Whenever you use wire wheels, keep in mind that the wire tips are designed to do the work for you. Too much pressure can bend the wires and prevent the tips from striking the material. You also want to use consistent strokes: Use smooth, even strokes rather than short, choppy ones when grinding. 

Pro Tips: 

• Due to their ability to aggressively remove material, angle and die grinders aren’t recommended for thin metal sheets. 
• The use of nonwoven abrasive products (e.g. such as surface conditioning or surface finishing discs) with nonferrous materials can cause issues, as they are typically made from loose nylon filaments impregnated with abrasive grain and can melt and “smear” when operated at high speeds, leaving a nylon residue on the workpiece. 

Red Label Abrasives: A Cut Above The Rest

Since a clean surface is so important for metal welding, you’ll want to ensure that you invest in abrasives that are expertly engineered, perform as expected, and have a superior lifetime cost of ownership. 

At Red Label Abrasives, we have provided steel fabrication companies of all sizes and production levels with sanding sheets, sanding belts, and sanding discs that strip away buildup and impurities and set the stage for a successful welding project. We carry a wide variety of grits, backings, and sizes in stock, and we can even make custom orders for special applications. For more information or to place an order, please call 844-824-1956 or fill out our contact form today.