Cleaning Old Figures

So today I was cleaning my second hand figures and realized how sticky they are from age. I'm wondering what would be the best way to clean the figures with a more permanent effect. Soap and water is sorta effective but not long term. They get sticky again after a month or so. If anybody has any suggestions I'd really appreciate it <3

The main ones that get sticky are the cast-off figures and the mini os-tan ones. Please help ;^;