I am sure the painting needs to be cleaned. I did some checking on how to do that.
Cleaning artwork is possible with a dry dusting brush to gently remove surface dirt, a damp sponge with a little soap and a Q-tip dipped in a gentle cleaner. Professionals typically use solvents like mild soap and water or specialized cleaning solutions for paintings.
Consider taking the painting to the dry cleaners. Have them dry clean the painting just the front side. Would the wax medium react to the dry cleaning chemicals? What about Dammar Varnish? It is water soluble. Take the risk of ruining it and get it dry cleaned.
If not dry cleaned, it could be cleaned with mild plain soapy water and a soft brush. Draw up water with a wet-vac brush attachment. Use an abundance of water.
Just now realize that I think the painting should be cleaned.
Ask a professional about technologies in cleaning Art.
45 PERCENT 