I needed to make a repair on some PVC pipe. When I opened my can of cement, I discovered that is was thicker than when new. I purchased the cement around a year ago and kept the lid firmly closed. The can is about 2/3 full. I could still easily spread the cement around the pipe. At what point is the cement no longer viable? Can it be made viable by thinning it with PVC primer? Yes, I should go purchase a new can of cement.
I use MEK (methyl ethyl ketone) available at most hardware stores. It has always worked for me and I've never had a fitting failure yet. I have had fittings fail from using thickened glue. Granted it's not the best thing to do, and I always buy a new can as soon as possible. But I do know where you're coming from when you need to do something to be able to finish a job, and can't run to a store at 3 am, or when you are 30 or more miles from any supplies.
Being that is the main ingredient in most welding adhesives or glues. Look at any MSDS Sheet and you will see the solvent used in it. For example: http://www.speedlinepvc.com/user/pdfs/5152-pdfs-5-file.pdf
I know I'm satisfied with my results. And storing that can upside-down always helps. I've even spread a bit of the glue around the top of the can to dry into a seal if I know I'm not going to use it for a really long time.
PVC cement works, in major part, by creating a chemical weld between the two pieces. If it has thickened, that means there's less solvent, which means it probably won't dissolve/weld into the two pieces as well. It may "work", but the joint is likely to be weaker than it would be with a fresh can. Whether it's good enough is hard to predict, and depends in part on what you're using it for.
Primer contains the same ingredients as Cement, except Cement also has PVC resin in it, so theoretically you should be able to add primer to cement to thin it.
Oatey Purple Primer contains Acetone, Cyclohexanone, Tetrahydrofuran, and Methyl ethyl ketone.
Oatey Regular Clear PVC Cement contains Tetrahydrofuran, Acetone, PVC Resin, Cyclohexanone, and Methyl ethyl ketone.
A little thicker or gummy? If it is gummy at all do not use it for a pressurized application. I have tried it in a pinch because I did not want to make the time killing run to the hardware store and the joint blew out two days later, it literally came unglued. You would have thought that it would blow out at initial introduction of pressure, right? No, it waited; with evil intent, until I was not home and flooded the yard and garage.
I applied it as directed, clean fittings and pipe plus primer.
Have you tried acetone? The PVC cement I use lists acetone as one of the solvent contained therein.