I 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 explain 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 waterbased 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 re-coating with a different topcoat
• making it easier to sand the first coat of finish
• providing a superior moisture barrier
Sealer Versus Sanding Sealer
|Not every sort of topcoat finish benefits from a first coat of sealer. Oil and oil-based polyurethane do not, while waterbased polyurethane definitely does.
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 the deeper pores shrinks proportionally more, causing the pores to show up again as depressions in a matter of months.
Different types of sealers are often specifically formulated for particular coatings and tasks. For instance, both vinyl and urethane sealers increase adhesion on some substrates, add moisture resistance, and block anti-oxidants. But choosing a sealer for your everyday work need not be a confusing task.
Fortunately, there is a universal sealer that will do everything on the list above, and is compatible with every wood and between all coatings. Made by Zinsser under the Bull's Eye® label, SealCoat™ Universal Sanding Sealer is a modified dewaxed shellac with a long shelf life. (Freshly mixed dewaxed shellac is an appropriate substitute.)
When to Use Sealer
|The author prefers to flood sealer on liberally by hand (top), then wipe off the excess with shop towels. Wear gloves for this procedure. Stearated sealer builds up quickly and sands easily — although it creates a lot of dust. One or two coats are sufficient.
Strictly speaking, any finish that forms a film on wood can be used as a sealer. Some coatings are so good at this task by themselves that they are called "self-sealing" finishes. Other finishes are not, and they benefit from special sealers.
Shellac and oil-based finishes, (including Danish oil, varnish, and polyurethane) work so well by themselves that they do not require any special sealer under them. Some finishers prefer to thin the first coat of these materials to make them dry quicker or sand easier, but that is strictly a personal choice.
Lacquer and waterbased coatings, on the other hand, work better over sealer. The right sealer will lock in contaminating oils and waxes, reduce the number of coats needed by preventing excessive absorption, improve adhesion, and reduce grain raising under waterbased coatings.
The wood also plays a part. With very dense woods, such as rock maple, you can usually omit the sealer. However, spongy or absorbent woods, like poplar, red alder, and most softwoods, can benefit greatly from sealer, especially under lacquer. The sealer coat envelopes the porous wood, preventing the first few coats of lacquer from being excessively absorbed.
Some problem woods, like rosewood and cocobolo, contain antioxidants that prevent certain finishes from curing. These need sealer under oil-based coatings, but not necessarily under lacquer or shellac. Fortunately, it doesn't hurt to use the correct sealer, so when in doubt, err on the side of safety.
You can apply sealer as you would any coating, with a brush, gun, or pad. The problem is that after one coat, end grain and spongy areas may still be "hungry" and insufficiently sealed, while denser flat grain areas are starting to build up too much coating. I prefer to flood the sealer on liberally by hand, using a nylon abrasive pad as an applicator, then immediately wipe it off with paper shop towels while it is still wet.
|The author's favorite sealer, Zinsser's SealCoat — a long-lived de-esterified shellac, works for all sealing applications.
Wear gloves and work small areas at a time so the sealer does not dry before you wipe it off. The advantage of this method is that it allows end grain to absorb as much sealer as it can, but wipes any extra off flat grain surfaces that tend to absorb less. Once the sealer is dry, the entire piece is uniformly sealed, and the next coat of finish will lay out the same in all areas.