Tinker Toys, Soccer Balls and Spaghetti

Using This Information

There are five characteristics you commonly 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 is a price to pay for protection and durability. Scratch resistance, for example, has the negative side of making a finish difficult to rub to an even sheen using abrasives. Solvent resistance means greater difficulty recoating, repairing and stripping. Heat resistance makes burn-in repairs less successful, and alkali resistance increases the difficulty of cleaning brushes and stripping.

At the sacrifice of better protection and durability, most refinishers and high-end furniture factories use lacquer instead of two-part finishes because of the reduced problems recoating and repairing, and because lacquer is easier to polish to a beautiful satin sheen.

Ease of Application

As a final lesson to be learned from the Tinker-Toy, spaghetti and soccer-ball analogy, consider that finishes are easier to apply the faster they dry and become dust and sag free. Fast-drying shellac, lacquer, catalyzed finish and, to a lesser degree, water-base finish, are less likely to run, sag or collect dust than varnish. If you are one of the many woodworkers who use regular alkyd or polyurethane varnish for your finishing projects, you need to recognize that you’re using the most difficult of all finishes to make look nice.

Ease of application is the reason factories that want the ultimate in protection and durability for objects like office furniture and kitchen cabinets use catalyzed finishes instead of polyurethane, even though these finishes are still difficult to rub to an even sheen and to repair and strip. No finish has everything.

Conclusion

It’s often difficult to keep all the characteristics in mind when choosing among finishes, especially if you haven’t used them all yourself. You can overcome much of your lack of hands-on experience by using the mental pictures of Tinker Toys, spaghetti and soccer balls to help keep the differences straight. PW

Bob Flexner is a contributing editor for Popular Woodworking.