If you’ve ever opened a very old tin or bottle of chalk paint then you’ll have experienced a bad smell. I’m not exactly sure why this happens but it’s the same with PVA paint. Maybe it’s the latex in the cha;k paint?
You can avoid this by using Milk Paint Powder. Milk Paint Powder has a very long shelf life and won’t smell when opened after sitting on your shelf for a few years.
But, if you do mix it then store it in the fridge. And then only for a few days. Because there skimmed milk powder is one of the natural ingredients it will turn after a time. So just mix as much as you need.