Put on your safety goggles and face mask. Sand the surface areas of the polyethylene that you are going to weld together until they are both rough to the touch.
Use the grinder to sand a 30-degree angle, or bevel, onto both pieces that you are going to weld together if you are going to weld a butt, edge or corner weld. Overlapping pieces or fillet welds do not need this bevel ground into them.
Wipe off the surfaces with a cloth to remove any debris made from grinding and sanding.
Preheat the area you are going to weld with the HDPE welding gun by holding it at a 45-degree angle to the welded area, slowly moving it up and down the weld joint area. Allow the hot air that the weld gun blows to hit the sanded surface of both pieces of polyethylene for two minutes.
Insert the sleeve of the welding spline guide onto the end of the polyethylene welding spline. Touch the guide end of the spline onto the preheated joint.
Hold the welding rod of the HDPE welding gun at a 45-degree angle to the polyethylene, about 1/4 inch to 3/4 inch above the material. Tack the polyethylene pieces together by placing small amounts, about 1 inch of melted spline, at both ends of the joint or gap between two pieces and in the center of the joint.
Hand-feed the weld spline into the joint of the polyethylene while taking care not to get your fingers too close as you may burn them on the hot material. The preheated polyethylene will make the spline start to melt.
When it melts, start to move the spline over to the next tack section of the joint. Insert a new piece of welding spline.
Place even pressure on the rod end of the welding gun, and when a bead of melted spline starts to form at the rod's end, begin to slowly move the welding gun up the polyethylene joint. Keep the bead visible along the edges of the welding rod as you go to ensure that you are placing enough melted spline into the joint to make it a strong weld.
Allow hot air from the welding gun to blow onto the polyethylene. As you weld, lay down the melted spline at a rate of 1 inch every two seconds, moving down the joint at a rate of 4 inches per minute as you start to fill it in.