How Varnishes Cure

A short but intense primer on the ways various varnish formulations harden and cure. December 31, 2005

Does anyone know the difference between a conversion varnish and a reactive varnish like polyurethane?

Forum Responses
(Finishing Forum)
From contributor M:
The two part conversion varnish uses a catalyst to crosslink the molecules, making it somewhat more durable and chemical resistant than the single component polyurethane.

From contributor B:
Conversions dry by solvent evaporation and by chemical reaction (crosslinking of the resins). Reactive varnish dries in a similar manner - solvent evaporation and a chemical reaction - but not crosslinking, rather, it's called oxidation. This process is caused by a drying agent (usually a metal like cobalt) added to the urethane resin to force it to dry (or oxidize) at a faster pace than if the dryer was not added.

From Paul Snyder, forum technical advisor:
The difference between them is the cure time. Both are cross-linking finishes that provide a durable finish once cured. The conversion varnish requires a catalyst which initiates the polymerization/cross-linking and causes it to cure more quickly than an un-catalyzed varnish/polyurethane.

From contributor S:
Let's be definitive. There are two types of coatings: convertible and non-convertible.

Non-convertible coatings do not go through a change when they dry and can be re-dissolved by their original solvent. Shellac, cellulose and thermoplastic acrylic are such materials.

Convertible coatings go through a chemical change (get converted) and are no longer soluble in their original solvent.

Within this, there are a number of different types of curing systems. There are the air-drying or oxidative resins such as alkyd and urethane oils that use oxygen from the air. This is a slow process that can be sped up by the addition of metallic soaps (driers). There are those coatings that will only cure when the conditions are right (acid catalyzed systems need to be in an acidic condition to cure, which is why they get upset on lye-treated wood). Finally, there are the true two-pack reactive coatings that need two specific components to react together to form the film. Epoxy and polyurethane are examples. All convertible coatings are reactive - it's just some have to have the reactive material provided, whereas others find it in the environment (oxygen for alkyd and oil-based systems, moisture for moisture-cured polyurethanes).

From contributor M:
Here in the USA, we refer to these two types of coatings as evaporative or reactive.