I have a gallon of Primer that has not been opened since I purchased it. I just read the sid and it says that it expired 5 years ago. Any idea if I can still use this?

I wouldnt use it. Primer is cheap. Id buy some new stuff just to be safe

Cheap! I paid $80. Thats 2 years of auto traders!

Any idea on what is most likely to happen? Is it likely nothing will happen? Would I know right away?

first off what type of primer is it??

If it has been sealed tight and in a reasonable climate it will most likly be just fine.

The test... open it up and mix the death out of it...I mean mix and mix and mix OR take it to a paint shop and slip them a few buck's to put it in the shaker

See if there is any chunks in it or any seperation after mixing - if not use it.

If it's a 2k product then get fresh hardener and fresh reducer - Don't use the old stuff!!

If you test it at a piece of sheetmetal, you will easily find out, if it's still good. It must dry completly, give a good surface and adhere to the metal.

If it was stored unopened in a good place (cool but not frozen)it is probably still good. You may add some solvent thinner if some has gone.