Varnish Treatment

The basis for French polish, shellac is a very widely used single component resin varnish that is alcohol-soluble. It is not used for outdoor surfaces or areas where it comes into repeated contact with water (such as around sinks or bathtubs).

Shellac resin is a brittle or flaky secretion from an insect found in the forests of Assam and Thailand.

By definition, drying oils, such as linseed and tung oil, are not true varnishes, though they accomplish the same thing.

Untreated or ‘raw’ oils may take weeks or months to cure, depending on the ambient temperature and other environmental factors

Typically, modern commercial varnishes contain an alkyd for producing a protective film. These are chemically modified vegetable oils which perform well in a wide range of conditions. They can be modified to speed up the cure rate and thus harden faster.

Other than acrylic and waterborne types, all varnishes are highly flammable in their liquid state as they contain solvents and oils.

Oil and polyurethane varnishes remain liquid even after evaporation of the solvent but quickly begin to cure, undergoing successive drying stages until they get hard. The drying and curing time is sped up by exposure to an energy source such as sunlight or heat.

However, as it penetrates less into the wood, even a thick film of ordinary polyurethane may de-laminate – fracturing if subjected to heat or shock – and leave white patches. This tendency increases with long exposure to sunlight or when applied over soft woods like pine.

‘Oil-modified’ polyurethanes, whether water-borne or solvent-borne, are currently the most popular wood floor finishes.