Painting - Thinning Zinser 1-2-3 primer

I have almost a full gallon of Zinser primer that has thickened a bit due to the can being open a bit around the edges. What is the recommended way to thin this?? Or should I just give up on the whole can and buy a new one?? I'm priming some MDF for use as utility shelving and it's a bit thick to apply easily. Thanks in advance - I'm new to this forum and this is my first post.
Connie

I generally don't recommend thinning primer, but you need to bring it back to life somehow. I would add just a tiny bit of water and stir real good. Or ideally, take it back to where you bought it and ask them to shake it again.

Thanks for the advice on the primer. I ended up reading the directions on the can (duh???) and it said to thin only with about a tablespoon or two of water per gallon if necessary. It worked fine and although the primer was a bit thicker than the new can that I had just been using, it worked fine and spread well using the foam roller. I had thought the can was closed tightly but there was a section where the paint had built up and was keeping the lid from sealing properly. I dug all the gunk out with a screwdriver and hopefully it's properly closed now. Lesson learned!!