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 chalk 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.