It would be a tragedy to damage a valuable painting by trying to remove the varnish if you have not had any experience in varnish removal. The best advice is simply to take it to a conservator.
However, there may be occasions when you feel you can remove the varnish on your own. The best product to use for this is Winsor & Newton Distilled Turpentine.
Dammar varnish can be removed using any of our three oil colour solvents: Distilled Turpentine, Artists’ White Spirit and Sansodor. Distilled Turpentine is recommended, as it has the strongest solvency.
Starting in a corner of the painting, dip a lint free cloth into the turpentine and gently rub the surface of the painting. The varnish should come off on the cloth – but remember to keep a close eye on the cloth and stop if you see colour coming off. Working in small squares, make your way across the surface of the painting and try to keep using fresh pieces of cloth, as this will help lift the varnish, rather than simply spreading it about.