Once the painting has dried, I know that acrylics dry almost instantly,but wait a week or so. Then apply a medium called a top coat, it is a permanent acrylic medium that is permanent and can not be removed, then varnish the painting. If the painting becomes dirty for whatever reason the varnish can be removed without harming the paint and then re varnish. Golden manufactures such a medium. As a matter of fact the Golden website is rich in tech. info on acrylic paint and the mediums and varnishes used.
Hope this helps, Cliff.