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I would like to paint varnished wood paneling. Should I sand and then put a coat and paint or put decorative plaster? I do not know how to do.

The varnish is nothing more than a generally solvent paint, without pigment or opacifying charge... If it is adherent, do not blister, do not crack, you can just wash it well, deglaze it the surface, then apply the solvent-based paint (eg glycerol) of your choice. For an aqueous, acrylic or vinyl paint, it will be better to strip the wood or apply a diluted undercoat, or a primer.

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