There are several issues here. First, you never have to sand down PVC pipe to make it fit. It sounds like you were trying to make the joint fit together dry, and that doesn't work. You need to understand how these joints are made. PVC joints are really glued at all, they are chemically welded. The so-called glue is actually a solvent that briefly melts the surfaces of the pipe and fittings. While in this semi liquid state, the pieces will slip together easily, and when the solvent dries (a few seconds) the joint is complete. You do need to avoid applying water or twisting the joint for a few minutes. There are 3 kinds of plastic pipes in use today. ABS used only for drain lines, CPVC appproved but usually not recommended for domestic hot and cold water lines, and PVC. PVC is used for underground cold water supply and irrigation pipes. Each of these three types use their own specific "glue" but they all do the same kind of joint. More specific to your question. Screw together unions are not what you want to use on your irrigation lines. There are a couple to connector than will work nicely and are simple to use. The URL below will take you to a site that has photos. These should be available at most any store that stockes PVC pipe. You will need to cut out the bad joint, use some short PVC pipe and regular couplers to get the pipe length restored before using this repair coupling. You will not have much chance of success using a plain coupler because the solvent sets up to quickly.