CAUTION: If a painting has been varnished before the paint has had time to dry completely, the paint under the varnish will remain soft for quite some time because the varnish layer seals the paint from oxygen, making it difficult to continue drying. In this case, when removing the varnish, the paint may also (partially) dissolve.
When removing old varnish layers, proceed with care and pay close attention to ensure that no problems occur during cleaning.
Step 1: Take a flat brush a few cm wide, dip it in white spirit (use only white spirit 090 for this purpose) and brush an area of about 15 x 15 cm.
Step 2: Wait until the varnish starts to swell, meanwhile rinse out the brush and repeat the operation over the swollen varnish. It will now partially dissolve in the white spirit contained in the brush.
Step 3: Keep repeating the operation with a rinsed-out brush until all the varnish on this spot is removed.
Step 4: Then treat the next piece.
NOTE: When the varnish is removed, there appears to be a white haze on the painting. This will disappear when a new coat of varnish is eventually applied.