"joevan" schreef in bericht
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Safest is to start with mineral spirits. Use a small bit to start on a
cotton skewer. If it was varnished with a dammar or such then the
varnish will come off with the dirt. If it was never varnished and
dirt has settled on the actual paint then carefully use some damp
cotton and see if that might help remove the dirt. We use a special
soap that is not harmful to the paint. A bit of enzyme might be
needed. That is in your spit.

Thanks joevan, you're the expert, I'll try.

I only say this because it is your own painting and numbers and all.
Just do a corner or so until you think it is ok.

I will.
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Greetings
Bouler (The Netherlands)