Polyurethane will not bond or grip well with the lacquer and will peel off over time with general use. It is meant for interior use and is made with alkyd resin cooked with one or more of the oils.
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I assume we are talking about oil based varnish.
Alkyd varnish over lacquer. Lacquer paints dry to a hard shell leaving a very thin paint film. Other varnishes such as polyurethane varnish dont gum up sandpaper so they sand easily without the need of a sanding sealer. Painting over laquer finished cabinets with alkydoil best primer hello cant seem to get a straight answer from my local paint storereps in confirming the best primer system to go over previously painted lacquer cabinets wood and mdf with a conventional oilalkyd ie satin impervo or glyptex.
Second polyurethane is not the best choice for painting over lacquer finishes. For these reasons lacquer is a good undercoat for any other paint product. Instead use an alkyd varnish.
There are some varnishes made with phenolic resin and also labeled varnish but these are rare. Alkyd varnishes are a polyester resin that will adhere better and repair more easily. Varnish including polyurethane varnish is meant for brushing.
As it happens regular alkyd varnish and nitrocellulose lacquer gum up sandpaper when sanded. It is always labeled varnish so far as i know. Alkyd varnish is the common varnish available at paint stores and home centers.
Traditional resin varnish alkyd or phenolic are less likely to have adhesion problems over other finishes than polyurethane varnishes though if the oilvarnish finish has had time to be mostly cured say a month the risk of. Lacquer and enamel top coats. Lacquer can be over coated with other lacquer paints or enamels and acrylics.
In fact sanding sealer will hinder the bond of polyurethane so you shouldnt use it. Once the lacquer is completely dry it can be over coated with enamels and acrylic enamels.
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