Aluminum extrusions are bars made with hollow cavities that allow each to be joined easily, using special connectors. The primary difficulty when painting aluminum extrusions centers on the fact that they are nonporous and durable. Each of these traits creates a complication of its own.Your question supposes that you need to Paint the Aluminum Extrusion. The more common thing to do is to Anodize the extrusion. This process coats all surfaces with a hard durable coating. There are several colors available. Actually It\'s common to use Urethane\'s to paint Aluminum and the primer isn\'t simply made for metal, but is specifically for Aluminum. A lot of Airplanes are coated every day in this manner. Methods for coating the insides of tubing have been used for a very long time. You basically fill the tube with primer or coating then drain the excess out (dip method) and difficult shapes can be powder coated or electro static coated. Extrusions can be roller coated after cleaning, vinyls work very well with out priming. Think Toothpaste tubes.Other related question