Woodworker's Journal 2011-35-Winter, страница 38

Sealer: The First Coat Of Finish

Our expert demystifies the options

and purposes of seal coatings.

suspect far more people are confused about sealer than understand it As a result questions abound, such as: "Is it the same thing as pore filler?" (No) "Should I use sealer before my topcoat?" (Sometimes) "Is there a universal sealer that will work with every finish?" (Yes) It's time to clear the air and explaki what sealer is, what it isn't, and when to use it

What is Sealer?

Sealer is the first coating applied to raw wood. While its use is always optional, it can be very helpful in some situations, performing all or some of the following functions: • locking in contamination on the wood surface, such as grease, oil, wax or sap and antioxidants that occur naturally in the wood

• preventing spongy wood from excessively absorbing repeated coats of finish

• reducing grain raising under water-based finishes

• acting as a tie or barrier coat to allow otherwise non-compatible finishes to go over one another

• increasing adhesion of the topcoat to the substrate or stain

• preparing an old finish for recoat-ing with a different topcoat

• making it easier to sand the first coat of finish

• providing a superior moisture barrier

Sealer Versus Sanding Sealer

If you've ever sprayed nitrocellulose lacquer directly onto poplar or cedar, you know these woods absorb quite a bit before the film starts to build. The hard lacquer is difficult to sand once you do

get enough on to seal the wood. To deal with this problem, sanding sealer was developed.

Sanding sealer is made by adding zinc stearate — a soft, fluffy soap — to lacquer. This causes the lacquer to build up faster, sealing spongy wood quickly. The stearated sealer sands easily and acts as a lubricant to boot Because this coating is softer than the lacquer that goes above it use only one or two coats. Building up a lot of sanding sealer can make the harder lacquer above it more prone to chipping and cracking.

Because sanding sealer builds fast and sands easily, some folks try to use it as pore filler, building up coat after coat and sanding it back until the pores are filled. This is not a good idea. Pore filler is inert, but most sealers continue to shrink over time, just as finishes do. The extra material in

Not every sort of topcoat finish benefits from a first coat of sealer. Oil and oil-based polyurethane do not, while water-based polyurethane definitely does.

38 sealer: the first coat

of finish