It all depends how long was it sitting there most paints that sit for a long time what happens is all the oils and tit rises up to the top there fore need to be shaken up really good to get it back to we're it should be.
Did you shake it?
KILZ smells bad and looks yucky when it's new. I'm with Mark... Stir, shake, stir, stir and stir...
Is your garage conditioned? The Kilz manufacturer's website has a big all-caps warning - KEEP FROM FREEZING - but then just says to thaw before use. I have seen other places say that freezing ruins it, but I couldn't confirm that through the Kilz people.
If it's oil-based, it just needs stirring but I have a hunch you have latex based that's soured over time; I too know the smell well
All paint should be stored in conditioned spaces. Good ol mildew will grow in latex paints when left in warm environments and of course it is not good for the paint to freeze either.
You have an interesting question. I don't know exactly what will happen if you don't re-kilz. A couple of times over the years, out of necessity, I have painted over it but then left, never to return.
Sad to say, if it were my house, I would probably need to redo the room.
Most folks make this mistake of storing their paint 'outside' in the garage.
Our garage is heated and rather cool in summer. But I guess it just got old and stinky. I painted over it - I hope it's ok. Thanks for all the comments - yes, shake stir shake stir stir stir.
Latex paint that has gone bad smells a bit like rotten eggs (to me anyway). Is that anywhere close to what it smelled like to you?
No your right Dan worse then year old eggs some times lol
Here is an article that gives tips for storing paint. It may help:
No, it doesn't smell as bad as rotten eggs. Thanks for the article Kass.
When in doubt, don't is my motto. Why use a product that smells to begin with? A fresh quart of tinted Kilz would have been better. Excellent question though.