I know the instructions say don't do it, but I had some drainage lines at my own place and my 721 was too thick. I thinned it with a little primer just to get enough to finish the job. There will be no pressure in the lines and small leaks won't matter if they happen and it was at my home. I just needed to get them cemented together, in the trench and the supplier was closed. It made me wonder, how many of you have thinned solvent cement with primer on irrigation projects and what were the results?