Conversion varnish, also called catalyzed varnish, is the most protective and durable finish commonly available to amateurs and small shops. In industry it is often used on office furniture and kitchen cabinets.
The finish is supplied in two parts: the finish itself, and an acid catalyst that has to be added just before application to make the finish cure properly. Conversion varnish thins with xylene, toluene or a related solvent specified by the manufacturer.
You can usually find conversion varnish at distributors or stores that cater to professionals. The trade off for the added performance is greater application difficulty.
The catalyst has to be added exactly as specified or problems will occur. Too much catalyst and an oily residue, called “bloom,” will develop on the surface, and the finish might crack. Too little catalyst and the finish won’t harden properly, and it may wrinkle when the next coat is applied.
Once the catalyst is added, you have a limited time specified by the manufacturer to use the finish or it will cure in the spray gun. You also have a limited time to complete the finishing so the coats bond well. As a result, if you’re doing any pore filling or glazing with slow-drying products, you may want to do these over a vinyl sealer and with vinyl washcoats between steps so you don’t exceed this window. Other sealers or finishes may cause wrinkling or cracking if you apply conversion varnish over them.
- The acid catalyst causes color shifts in many dye stains, so always use the manufacturer’s recommended stains.
- Due to the hardness attained, you shouldn’t apply more than about three coats of conversion varnish or the finish will crack – sometimes months later.
- The temperature should be kept in the upper 60s or higher for two days after application, or the finish may not cure properly.
- Repairs to conversion varnish are difficult to do well enough so the repair doesn’t show.
When spraying, it’s very important to work in a quality spray booth with good exhaust and use a proper NIOSH-approved respirator.