Removing polyurethane varnish

I am currently investigating the possibility of removing a strongly
discolored varnish from a painted polyurethane hard foam sculpture
(1967). The paint used is an alkyd paint which has been applied in
several layers. These paint layers are covered by a roughly applied
transparent polyurethane varnish. This uneven varnish layer has
darkened (turned brown) considerably over time, making the
unevenness and drip marks more evident and changing the hue and
uniformity of the colors.

A solution is sought in the difference in solubility of the PU
varnish and alkyd paint layers, but the solubility range of both
materials are not known. Up to now, Shellsol A, methyl ethyl ketone,
iso-propanol, several acetone/white spirit mixtures and acetone gel
(acetone in Klucel G) have been tested under a stereo microscope.
Some of the tested solvents and solvent mixtures were able to
slightly solve the PU varnish, but eventually caused swelling and
damaging of the underlying alkyd paint layer due to mechanical
action.

I would like to know if someone has any experience with the removal
of a forty year old polyurethane varnish, especially in combination
with an underlying (alkyd) paint layer, or has any suggestions on
how to go about?

Evelyne Snijders
Stedelijk Museum Amsterdam
+31 20 5732674


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                  Conservation DistList Instance 22:27
                 Distributed: Sunday, November 9, 2008
                       Message Id: cdl-22-27-016
                                  ***