Clear topcoats

Basics on post- and pre-catalyzed finishes and more. January 16, 2002

I am new to the field of clear topcoats. My main concern is durability and scratch resistance. I do not have spray equipment--I use various brushes and paint pads. Can you cover some basics for me?

Forum Responses
Get a copy of Bob Flexner's book "Understanding Wood Finishing." It has detailed info on the various finishes and stains.

Jeff Jewitt has a couple of good books on hand applied finishes that will help you get good results without spray equipment.

Most lacquer paints are durable, to a point. The most durable are the "post-catalyzed" (conversion varnish) finishes. However, they are meant to be sprayed on, not brushed on. If maintained properly, you can use a variety of spray equipment, anything from a quart cup set-up to the siphon systems. Post-cat products are to be catalyzed at point of spray--that is where you get the durability.

Most people do not like the measuring involved or the cleaning of the equipment, so more often they choose the "pre-catalyzed" products. I was in sales at an Atlanta-based company, and we sold more pre-cat, more convenient for customers (no mixing of catalyst) and lower in cost.

I often said "if I was an everyday spray person I would choose a conversion over a pre-cat any day", even with the mixing involved, and the daily clean up of equipment. There are a lot of brands on the market. Some are HAPS compliant, which are better for the consumer. I am only familiar with ML Campbell and Gemini. Both companies have a great conversion varnish. Check with suppliers in your area to see what they offer.

Lisa Gilbert, forum technical advisor

Always begin with the end in mind. Your end-users are children, so your furniture will need to withstand a lot of abuse. Besides, your choices are limited since you are not set up to spray solvent-bornes.

My advice is to look into using a 2-component waterborne floor finish. A WB finish is comprised of fully-cured independent molecules suspended in water. As the water evaporates, these independent molecules begin to move closer and closer until they connect with one another.

The 2-component kit comes with a catalyst. Once activated, these products will have a pot life of approximately 24 hours, so you may only want to catalyze enough material to apply in one day. The catalyst acts just like a cross-linker and starts a chemical reaction once you intermix it with the urethane. It will enhance the fusion of those independent molecules, making the overall dry film properties a lot more durable.

Once applied, the film will cure in 7 - 10 days. Most WB urethanes are aliphatic by nature, meaning that they dry crystal clear. Ideal for use over pickle white stains, but do nothing to add warmth or depth to the appearance of the wood. Waterbornes are designed to be applied at 2 mils wet. These products roughly have 30% weight solids, so your overall dry film thickness can be low. This means that at least 1 sealer coat, followed by 2-3 topcoats, will be required for maximum protection. WB also lack the necessary solvents to partially melt into existing coats the way lacquers do, but you can re-coat within 2-3 hours of good drying (WB dry times will vary based on the environment they are applied in).

Expect some initial grain raise when working with WBs. If you are going to leave the wood natural, you might want to pop the grain with a mixture of water and isopropanol first, let it dry, and then scuff sand with fine paper. This will minimize the initial roughness from your sealer coat and increase your initial build and wear layer.

Do not use white oak and waterbornes together due to the tannins in the wood unless you find a specific sealer that will act as a barrier coat to prevent the possible discoloration that will occur.

Never apply more than 2 coats of catalyzed finish in one day. Let your sealer dry overnight. Never use steel wool between coats, let solvent-borne stains dry for at least 72 hours. Remove all sanding dust and scuff sand between coats to promote proper adhesion.