The Way Finishes CURE

All the products used in finishing cure by one of two types – evaporative and reactive. Each type cures differently. A finish cures by changing from a liquid to a solid on the wood.

EVAPORATIVE FINISHES

Evaporative finishes are made-up of solids that have been dissolved in a solvent. These are made-up of long, strand-like molecules that pack together and unite when all the solvent present in the solution evaporates. A thin solution can be achieved by increasing the solvent dilution which also means the less the solvent added, the thicker the resulting solution. When all the solvent evaporates only the solid contents are left. These solids are the same as what they were before they were dissolved. They then transforms from solid flakes into a film on the wood.

The strands are intertwined when soft and they’re interlocked and form a continuous film when dried; but nothing bonds the strands together, re-introducing of solvents will soften the strands. Evaporative finishes can change back and forth between liquid and solid by the introduction or evaporation of the solvent. 

Applying a coat of evaporative finish over an existing cured coat will dissolve the underlying coat. The solvent puts the strands back into a solution, so the coats fuse making one thicker layer. 

Evaporative finishes cure from the bottom up. The solvent at the bottom travels through the film layer which then makes the top layer the last part to cure. When the top becomes hard, you can be sure the film is cured all the way through.

REACTIVE FINISHES

Reactive finishes crosslink when they cure. As the thinner evaporates, the resin molecules move closer together resulting into a chemical reaction that links the molecules together forming a network. Reintroduction of the thinner doesn't break the bonds that are formed. This chemical reaction is referred to as crosslinking or polymerization.

Reactive finishes don't bond chemically between coats once the previous coat has cured. The film formed won't fuse to the previous coat. Making fine scratches by sanding promotes the new coat to lock or adhere mechanically. Also, sanding dusts must be removed because it won't be dissolved into the new coat. 

There are two categories of reactive finishes those that cure by reacting with oxygen and those that cure by reacting with a chemical catalyst. 

For oxygen dependent curing, the application of coatings must be kept thin. Oxygen makes contact with the external part of the coating then through the bottom of the film. It cures from the top down. Once the top cures and forms a skin, oxygen is inhibited from penetrating to cure the rest of the coat. Thick coats take much longer to cure all the way through.

For catalyst curing, it cures from the bottom to top. The solvent that carries the material will travel from bottom to top; which then allows the molecules to crosslink.

COMPARISON

The main difference between EVAPORATIVE and REACTIVE finish is whether or not the molecules are cross-linked. Crosslinking makes the produced film more resistant to heat, scratch, solvents, acids and alkalis and making it less penetrable. There's little space between cross-linked molecules for liquids or water vapor gases to pass through.

The strong bonds of cross-linked molecules produce a durable film but for the same reason the coat produced is difficult to repair. A new coat of finish won't dissolve the previous coat; a visible line will separate a repair from the original finish. There are few solvents and chemicals that can dissolve the film.

Evaporative finishes are not cross-linked, they are easiest to scratch, dissolve and penetrate. An advantage with evaporative finishes is that it would be easier to repair since a number of solvents and chemicals can separate the molecules. A disadvantage would be that the un-cross-linked molecules have large spaces in-between that results to water and water vapor penetration.