Popular Woodworking 2000-10 № 117, страница 15

monly look for in a finish.

• Protection for the wood (resistance to water and moisture-vapor penetration);

• Durability of the finish (scratch, heat, solvent, acid and alkali resistance);

• Rubbing qualities (ease of rubbing to an even sheen);

• Reversibility (ease of repairing and stripping);

• Curing speed (ease of application without dust or sagging problems).

The Tinker-Toy-like reactive finishes - varnish and two-part finishes - are very protective and durable because the cross-linked molecules are difficult to penetrate or break apart. (Oil finishes offer very little protection or durability because they cure too soft and are left too thin on the wood.)

In contrast, the evaporative finishes -shellac and lacquer - allow some water and moisture-vapor (humidity) penetration through the gaps where the spaghetti-like molecules bend around each other. And these finishes are easier to damage with coarse objects, heat, solvents, acids and alkalis because their molecules aren't held together by the strong ties common to reactive finishes.

(Like oil, wax is considerably weaker than shellac and lacquer because it's too soft and left too thin on the wood.)

The coalescing finish - water base - is resistant to abrasive damage because almost all the surface area is cross-linked inside the soccer-ball-like droplets. But where the droplets stick together, water and moisture vapor can penetrate; and heat, solvents, acids and alkalis can cause the droplets to separate.

This is not to say that evaporative and coalescing finishes are weak finishes, only that they aren't as protective or durable as reactive finishes. If you want the best finish for a kitchen table, kitchen cabinets or office desk, it's a reactive finish (not oil). But this degree of protection and durability is seldom necessary for an entertainment center, most woodwork or a bed.

The Trade Off

No finish can provide it all, however. There continued on page 26

12 Popular Woodworking October 2000