The problem you are describing is a common one. Let’s take a moment to see what anodizing actually is so we can understand the problem better. Anodizing, a relatively thick coating of aluminum oxide, is performed by making the part the anode in an electrochemical cell. It’s a bit like plating, but in reverse. Instead of depositing a metallic plating on the part, you deposit an aluminum-oxide coating. The result is a relatively clear coating of aluminum oxide. The coating’s thickness varies depending on the anodizing solution and the parameters you use, but it is much thicker than coating that occurs naturally, and it is relatively porous. A brightly colored anodized coating usually means the color was added after anodizing by immersing the part in an organic dye. The anodizing is porous enough so that the dye penetrates it and remains. When you try to weld over the anodizing, you will typically experience two problems. First, the anodizing is an electrical insulator that makes arc-starting difficult. It also acts as a barrier to the flow of the molten weld—remember that aluminum oxide melts around 3,700 degrees F, while the aluminum melts around 1,200 degrees F—so the weld pool will look gummy and uneven. Second, the anodizing is porous and absorbs water vapor from the air so when you weld over it, you will get a considerable amount of porosity. So what should you do? Well, the easiest thing is to remove the anodizing in the area that you’re welding using a sanding disc or grinding wheel. If you can’t do that on the basis of weld appearance, there is a technique used by a small number of fabricators specifically for welding over anodizing. First, set your AC GTAW power supply up near its maximum current capacity—let’s say 300 amps. Now use a foot pedal to turn the arc on at 300 amps. As soon as the weld forms, add a drop of filler metal. After a second or so, turn the arc off. Move the torch ahead a bit and continue to repeat this process. In reality, you are making a line of overlapping spot welds. This procedure can make a beautiful weld on anodized material, but it takes a while to learn and master the technique. You might want to start practicing now.