Waterproof Varnish

Waterproof Varnish (326), pg. 258 in book Lost Secrets

{marginal notes] N.B.

M. Portsmann, Flemish painter believes that all varnishes, like Mastic, sandarac, or other gum resins which do not stand water without turning white and spoiling, can nevertheless be used without drawbacks, if a little greasy oil that has been bleached in the sun as said above is added to the varnish. This is dissolved and made fluid with spikenard oil which evaporates easily, so that the oil conserves everything remaining.
T.M. Try wheter thickened oil, or oil prepared with lead monoxide, or oil simply boiled down by half can be bleached with vinegar, water and alum, or with water or vinegar in the sun like simple linseed or nut oil which has not been put in the sun.