I left a 10ltr tin of smooth masonry paint outside last year I went to open it yesterday and discovered a crack in the lid,and when I opened it it was full of foul smelling water sitting on top of the paint I poured off as much as I could but needless to say some went into the paint, if I paint my walls,will the smell go away or is this paint beyond saving?look forward to your advice. I have heard white vinegar could help but not sure..
TLDR: Don't use dodgy or bad paint, it will cost you way too much in your time to fix the result.
The foul-smelling water is part of the paint, and you just wrecked the paint. It is foul-smelling because it contains things other than water, which perform a necessary function to curing the paint. It won't work without it. It is normal for paint components to separate out, leaving watery stuff at the top and solids at the bottom. So your reflex should never be to "dump it off".
Although realistically, if you had a crack in the lid, the essential volatile chemicals in that paint will have dried out regardless, even if they were replaced by rainwater -- it is now no longer functional paint.
Also, do not leave paint where it will freeze or get hot from the sun, and if you did, throw it out because it will not be reliable.
Alternately, you have low-VOC latex paint which has turned moldy (the VOCs, now outlawed, did a pretty good job of arresting mold). Anytime you get stinky paint, Do Not Use It because the stink will not go away. We have plenty of questions like "I painted my house with stinky paint, now my house smells disgusting and it won't go away, I've tried everything, what do I do?" The solutions are not easy. In most cases, their motivation was to save money.
People forget the big cost of a paint job is not the paint.