What preparation is needed before applying varnish?

If you’re applying over an existing varnish, ensure the surface is sound, if not the coating should be removed. Test for compatibility – To test if an existing topside paint product, or a varnish, is compatible with our two-part polyurethane products; tape a cloth soaked in Thinners No.9 to the previously painted substrate for 24 hours. If the surface is softened, it is probably not compatible. In this instance, unless stripping down to the bare substrate is an option, a one-part product should be applied. For bare wood, unless you’re using a clear surface primer, such as Interlux Clear Wood Sealer Fast Dry, we recommend thinning the first coat of all our varnishes. This promotes good penetration of the surface and adhesion of subsequent coats. After this first coat, the surface will appear rough – as the exposed ends of the wood absorb the varnish and lift – sand this smooth with a medium grit sandpaper.