TGIF: Identifying a finish Tuesday, Oct 10, 2017

You have an old piece of furniture (or gotten one).  You might be able to restore the finish by applying more of same (after a cleaning)

As a general rule,  shellac was a common finish up to WWI.   Lacquer started to take over and by WWII is the most likely factory-finish.   Starting in the 1960s, polyurethane became a common choice for Joe Garage.   All this time, varnishes of various sorts were used, but were uncommon for factory finishes. They simply cure too slowly and are difficult to repair for transit or showroom damage.   Around 2000, a few conversion finishes started to be used by factories.   These are very tough and don't respond well to top coating, touch up, or stripping.   Durability and repairability are opposite sides of the coin.

Oil or oil-varnish blend pieces are renewed by adding more of same.  These finishes are generally not "film forming" also known as "in the wood" and identifiable by observation.

Because of the nature of the solvents on the finish, the order of testing is important.

1. Alcohol
   Apply a few drops of denatured alcohol to an obscure place.  Wait a bit,  then touch the spot with a finger-tip, soft-bristle brush or a cloth. Shellac will l soften and turn a bit sticky. If it doesn't, it's not shellac.   Really old lacquer may also soften just a bit with alcohol.
2. Lacquer thinner
   Next, try lacquer thinner, and repeat the first step by applying a few drops of it to a new spot on the surface. If after a couple of seconds the finish softens a lot or becomes runny, you have lacquer. But if the finish only becomes tacky and you know that it was built in the last decade the finish could be water-based.
3. Zylene (Xylol)
   Try touching a bit of xylene to a different part of the finish. If the test area gets gummy, it's a water-based finish.
4. If none of these solvents dissolves the old finish it is a reactive finish that cures via chemical reaction, such as a varnish.

You can apply more shellac and lacquer over themselves.  Or you can clean up a bit by padding with some of their respective solvent.  Not too much or you'll strip down to wood.   This is the essence of the "finish refinishers"  See  ATM strippers under http://thepatriotwoodworker.com/topic/20769-tgif-stripping-old-finishes-tuesday-sept-19-2017/

You must thoroughly clean and dull (scuff up) film finishes before you can apply another coat of the same finish.  You especially need to remove dirt and wax for good adhesion.   See http://thepatriotwoodworker.com/topic/20685-tgif-saving-a-finish-tuesday-sept-5-2017/

Edited October 10, 20178 yr by kmealy