Here is an old formula that is both a wood finish and a great cure for blemishes on many kinds of wood finishes, including polyurethane and lacquer. I use it to treat scratches on hardwood floors, and furniture to make them virtually disappear. A scratch, scuff, or scrape is usually a mar in the surface of the finish that is on top of the wood. A deep scratch will mar the actual wood itself. The blemish on the finish usually shows up as a lighter area as the finish slightly detaches from the wood. What this 'sauce' does is to make this lighter area that is opaque turn more translucent and help to hide the blemish. On deep scratches it will also darken the marred wood slightly, further helping to hide the affected area. Test this on a small portion of the area first to determine if this is the fix that you want to go with.
The Recipe:
1 Part pure Tung Oil - This is a naturally produced oil that hardens over time similar to
polyurethane. **
1 Part Turpentine - This is a natural solvent made from Pine.
1 Part Polyurethane - A man made product that is often used as a sole Top Coat finish.
Spar varnish can also be used here, but limits the amount of time that this can be
stored.
Mix all ingredients in a new container. Empty 'paint cans' can be purchased at the paint counter of your local hardware store. Use a soft, clean rag to liberally apply the 'sauce' to the affected area, working it in till the blemish is reduced. After a few minutes use a fresh clean rag to wipe off any excess from the blemish and the surrounding areas. It should be dry in 24 hours. A second coat can be applied if needed.
This 'sauce' will soak into the exposed wood and surrounding finish as it dries. Applying more coats will build up the layer. In this manner it can be used as a finish coat on raw or sanded wood. I have used it to re-finish dining room tables, the trick being to use the fresh clean rag to wipe up any excess before it begins to dry (usually within about 5 or 10 minutes). Be sure to light sand between coats with 180 - 220 grit sandpaper after it is dry and before the next coat.
** This works best with pure Tung Oil. The 'Tung Oil Finishes' sold at the big box stores doesn't work as well as these have many other ingredients in the product. I suspect that Boiled Linseed Oil would also work in place of the Tung Oil, however, I have not actually tested this to be sure. I use Tung Oil often on my own projects and prefer the finished color to Boiled Linseed Oil, which is a bit more yellow in finish. Also note that Linseed Oil that is not boiled does not dry to a hard finish.