(In fact, sanding sealer will hinder the bond of polyurethane, so you shouldn't use it.) Think of sanding sealer as a production tool. If you're finishing large objects such as a set of kitchen cabinets, it will speed your work. Sanding smooth will be easier before applying the topcoats of varnish or lacquer.
System Three® Marine Spar Varnish can be used over some sanding sealers but it will depend on the formulation of the sealer. Most lacquer-based sanding sealers are not recommended because the sanding aids are wax-related.
Sanding Sealer is not recommended as a final finish because it is not as durable as a topcoat. Seal Sanding Sealer with any General Finishes water-based topcoat, EXCEPT General Finishes Enduro- Var, for improved water and chemical resistance.
It's a good idea to do this fine sanding between coats anyway to remove dust nibs. But polyurethane doesn't bond so well over finishes marketed as sealers, especially over sanding sealer. This sealer is good for use under non-polyurethane varnishes because regular alkyd varnishes gum up sandpaper.
Do not apply stain to the top of the sanding sealer. To work properly, wood stain needs to soak into the pores of the wood. Sanding sealer negates this process. The stain will sit on the surface and not be absorbed.
Think of sanding sealer as a production tool. If you're finishing large objects such as a set of kitchen cabinets, it will speed your work. Sanding smooth will be easier before applying the topcoats of varnish or lacquer. But on smaller objects you could just use the finish as the sealer coat.
There's a common misconception that wood must be “primed” with a special sealer before you apply a finish. The truth is that the first coat of any finish will serve as a “sealer.” However, it's important that this first coat can be sanded easily to provide a smooth surface for subsequent finish coats.
A Sealer is a mixture of oils and resins as well. However, it produces low viscosity as compared to varnish, which simply means that the sealer practically penetrates in to the wood to nourish and protect not only the outside but more importantly, the inside.
The thin viscosity makes it easy to spread and allows it to penetrate into porous woods. To start apply 1 thin coat and allow it to dry. Lightly scuff sand to remove any raised grain fibers, then apply an additional 1 to 2 coats to seal wood and provide an excellent surface that will be ready to topcoat.
Expert Tips For an Extra Smooth Finish:
A sanding sealer is not the same as a wood conditioner. Whereas a wood conditioner is a pre-stain treatment that reduces blotchiness when staining, a sanding sealer is applied only to bare wood that is not going to be stained.
Michael Dresdner: "Sanding sealer is never mandatory. It is an option that saves time and material in some situations, and is unnecessary in others. Some woods, like cedar, spruce, and poplar, are so porous that they tend to suck up the first few coats of sprayed lacquer as if nothing were applied.
Minwax® Professional Formula Sanding Sealer is a clear sealer formulated for application over bare wood. Its quick dry feature allows for sealing and topcoating your project in just one day.
Lightly sand the surface with 220-280 grit sandpaper. A powder should develop when sanded. Do not over-sand to avoid removal of the sanding sealer or previous stain.
Ingestion May cause nausea, headache, dizziness and intoxication. Skin contact This product is rapidly absorbed through the skin and may cause symptoms similar to those of ingestion. Repeated exposure may cause skin dryness or cracking. Eye contact Causes serious eye damage.
Sanding multiple times and applying more coats may take a lot of time and money. But, if you skip this process, the polyurethane coat will not cling to the previous coat, and the last coat will eventually peel off, causing you to buy more and start the project again.
The main difference between an undercoat, sealer, and primer is that primers are applied to a new surface, while sealers are used as either the substitute to a primer or they are applied before the primer. An undercoat is typically used on a surface that has already been painted.
Regular readers will know that we do not recommend over-thinning the Cellulose Sanding Sealer; it's a common myth that it needs to be thinned 50/50 to work better, but as a general rule this is incorrect.
In fact, finishes that cast a film on wood can work well on their own without a sealer underneath. Some good examples are shellac and oil-based finishes like polyurethane and varnish. But lacquer and other water-based finishes need a sealer under them so that they don't get excessively absorbed by the wood.
Use 120-grit sandpaper to smooth the surface. This will help the varnish stick and will give you a smooth finish. If you removed the previous coat with sandpaper, you don't need to do this step. Thoroughly stir the varnish so it's a smooth consistency with no lumps.
We recommend finishing your wood with oil-based stain (see “Test the Stains”) and wood varnish (alkyd) with a compatible sanding sealer beneath it. These finish types are the easiest to apply. Don't confuse alkyd varnishes with polyurethane ones.
Just before applying the final coat, sand lightly with 320-grit sandpaper and remove residue. The same goes for matte or rubbed effect finishes.