Sanding sealer just might be one of the best painting products that you've never heard of. Once you've used sanding sealer, you'll always want it on hand for prepping difficult porous materials before clear-coating or painting.
When you use sanding sealer, projects that formerly required multiple coats of paint—and which still came out spotty—go faster and produce better color consistency. Plus, projects can be finished in a day rather than spanning many days.
What Sanding Sealer Is
Sanding sealer is a clear liquid base finish that is applied to bare natural wood or to any uncoated wood product. Sanding sealer always must be sanded down smooth after it has dried.
Sanding sealer helps improve the condition of subsequent coats of paint or clear finish by sealing the wood pores. With the wood pores sealed, the coating is able to spread out and cover the surface rather than being quickly soaked up. Typically drying within an hour, sanding sealer helps wood achieve a smooth, even, and color-consistent top finish.
Uses for Sanding Sealer
- Painting medium-density fiberboard (MDF)
- Painting OSB
- Sealing wood knots
- Applying to furniture and cabinets
- Flattening out raised wood grain
- Painting any species of wood
- Sealing pores for smoother finishes
- Coating bare hardwood floors before topping with oil or polyurethane
What Sanding Sealer Is Made From
Polyurethane and shellac are clear coatings that are sometimes used to seal wood pores prior to coating. Sanding sealer is similar to polyurethane and shellac, with one major difference: zinc stearate additive.
Zinc stearate is added to the sanding sealer to help it produce a higher, fuller build. This gives you more material to sand.
The zinc stearate also acts as a lubricant to reduce drag when sanding and to help pulverize and slough the sanding sealer into dust. Polyurethane and shellac sometimes gum up the sandpaper; sanding sealer does not.
Some forms of sanding sealer are water-based, so clean-up can be done with mild soap and water. Any oil-based polyurethane or shellac must be cleaned with paint thinner or acetone.
When to Use Sanding Sealer
Sanding sealer should be used after the wood has been sanded down to a bare finish but before the topcoat (such as paint) has been applied. Applying sanding sealer to a stained surface is generally not recommended, as the sanding step will scuff away the stain.
Sanding sealer is a toxic substance and should be handled with care. Use only in well-ventilated areas. Methoxymethylethoxy propanol is toxic and can be absorbed through the skin, so wear latex gloves and protective clothing.
Since the point of using sanding sealer is to create dust, wear breathing protection. Sanding sealer creates fine, airborne dust that can irritate breathing passages.
How to Use Sanding Sealer
With a hand sander or oscillating sander, sand down the wood to a bare surface.
While sanding sealer will help smooth down the surface, you should get the bare wood as smooth as possible to start with.
First, use a shop vacuum, then clean off the remainder of the wood dust with a tack cloth or with rags.
Mix Sanding Sealer
Gently mix the sanding sealer with a paint stirring stick. Do not shake the can, as this will cause the product to develop bubbles.
Apply Sanding Sealer
Dip the paintbrush in the sanding sealer. Completely cover the wood surface with a thin coat of the sanding sealer. Do not apply a second coat at this time.
If desired, you can try a technique known as grain popping at this point: Apply a damp cloth to raw sanded wood to enhance the grain and open up the wood pores to better receive the product you're about to apply. If you want to try grain popping, do so before you apply the sanding sealer.
Sand Down Sanding Sealer
After letting the sanding sealer dry for at least an hour, sand it with #320 fine-grit sandpaper.
Clean off the sanding sealer dust with the shop vacuum and the tack cloth or clean rags.
You can immediately paint or clear-coat the surface after cleaning off the dust.
Tips for Using Sanding Sealer
- When sanding down the sanding sealer, be gentle. The aim is to sand down the product smoothly, but not to sand through to bare wood.
- If you do happen to sand through the sealer, repeat the process. Clean the wood, apply a second coat, and sand again.
- 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.
- If you're having a problem with your water-based sanding sealer raising the grain too much, switch to an oil-based sanding sealer.
"Dipropylene Glycol Methyl Ether ((2-Metholymethylethoxy)Propanol)." U.S. Department of Labor/OSHA.