Before appying to anything prepainted I would try it out on a test piece to make sure there is no reaction to the paint . I have never applied it over painted surfaces before . just bare wood . Some paints will have a reaction with others . Just check it out on something first . It would be bad if you did everything and thenn had a reaction to the one area that was painted . It should be fine though . Also on the varnish you got ,it will have to dry between coats . Ususally put on one coat per day . That will allow it to dry well before the next coat . And you will be able to sand (2000 grit) between coats if you have any errors in the varnish . The best thing is take your time doing it . Don't get in a rush , its a slow process to get that ultimate shine !