Real Milk Paint ® will not stick to oil base finish. Minwax stains, varnish, enamels and polyurethanes are all in an oil base. The way to determine if a finish is oil base is to examine the instructions for the recommended thinner or clean up procedure. If the instruction recommends mineral spirits or naphtha, the finish is oil base.
To make the milk paint stick to the oil finish you need a barrier coat. A barrier coat is a coat of finish between the milk paint and the oil finish. Before application of the barrier coat the surface should be cleaned with TSP (trisodiom phosphate) to remove any silicones or waxes, then scuff sanded for good mechanical adhesion.
There are three types of barrier coats we recommend: Ultra-Bond mixed with the first coat of milk paint , Zinsser Bulls Eye 1-2-3 Primer / Sealer or dewaxed shellac. Each will give different results.
Ultra-Bond is mixed with liquid Real Milk Paint ® for the first coat. Successive coats can be painted with unadulterated (pure) milk paint. Mix 25% Ultra- Bond to 75% liquid Real Milk Paint ®.
Real Milk Paint ® will stick to cured latex (30 days). If the latex is not fully cured crackling will occur. This is a nice reaction if you like crackle.
Real Milk Paint ® will stick to dewaxed shellac. Dewaxed shellac is generally only available from mail order companies. The shellac available at the hardware store has wax. Milk paint will crackle and slightly flake over the waxy shellac. This gives a nice antique over painted look.
Zinsser Bulls Eye 1-2-3 Sealer / Primer. It also has titanium dioxide (pigment) and calcium carbonate. You can get this tinted to light shade of the milk paint you may choose to use. Tinting is better than just painting over white for darker colors of Real Milk Paint. Once painted with Bulls Eye 1-2-3, then the first coat of milk paint should mixed with Ultra Bond. The second coat can be unadulterated milk paint. This procedure in likely the best for old plaster wall or surface that have seen multiple unknown uses. Zinsser Bulls Eye 1-2-3 Primer Sealer is very good at sealing out silicones and unknown contaminated. We used this exact procedure in our 1892 house with horsehair plaster walls.