Varnish can make a wood veneer lustrous and durable, but it can crack and weather over time. Removing old varnish from veneer is a delicate process. Veneers are generally thin strips of wood laid over a base wood, and some removal methods might damage the veneer or cause it to peel up from the base wood. Stain can be more difficult to remove, especially if it has seeped into the veneer.
Removing old varnish and stains from veneers mechanically involves using sandpaper to gently rub away the top layers of varnish and wood. Sanding is a good technique if there is a thicker layer of varnish or clear sealant over the veneer, and sanding away the top layers of veneer can also remove a stain. However, sanding can also damage thin veneers, and it will not be able to remove stain that has soaked past the first layer of wood. For thin or delicate veneers that might be damaged by sanding, a chemical varnish stripping process is better.
Since many varnishes are made from types of plastics, they will melt under high temperatures. To remove old varnish or polyurethane sealant, hold a heat gun over the surface of the wood until the varnish bubbles or begins to turn liquid. Scrape the varnish off using a steel scraper. Use fine sandpaper to remove any remaining varnish and create a porous surface for the new stain or sealer. Always follow safety procedures with heat guns and wear protective gloves.
Chemical Wood Strippers
Chemical wood strippers are powerful solvents that dissolve away old varnish and stains from wood. Most chemical strippers are made of additives and methylene chloride, a chemical that is toxic. To use a chemical stripper, simply apply an even layer to the wood and wait according to manufacturer's directions, then wipe away the stripper. Wood can then be cleaned with mineral oil or rubbing alcohol. Some chemical wood-stripping compounds can also warp or damage veneers, so always read the labels thoroughly before using a stripper on a wood veneer.
Another way of removing stains and varnishes that have soaked into the wood is with a process called acid bleaching. In this process, a bleach mixture is first applied; these chemicals break down bonds within the stain or varnish, releasing them from the wood. Then an acid compound is applied that removes the bleach and wood stain remaining. The acid compound must be thoroughly removed or it can eat into the veneer and leave marks.