Varnishes in the Artisan Water Mixable Oil Colour range can be used on top of conventional oil colour. As with all varnishing, ensure that the painting has been allowed to dry for a minimum of six months first.
It may be best to give the painting a light rub over with Artists’ White Spirit before applying the varnish. This will clean any grease or grime from the surface. If there is any trace of grease on your painting when you varnish it, the varnish will reticulate into beads where the grease is. Cleaning the work first ensures that the varnish goes on well.