Applying polyurethane – The finishing touch
After you’re done staining your pine wood project, applying polyurethane is the final step in achieving a beautiful, flawless finish.
Stain gives the wood character and color, but it’s not much of a protective finish. If you left your stained project as is, it would be susceptible to water marks, moisture damage, nicks, scuffs, and the like.
That’s where a good polyurethane varnish comes in.
Applying polyurethane to your project is like giving it a tough plastic outer shell. It will be impervious to water, most liquids, and moisture while also offering some heat protection and scuff resistance.
And polyurethane can add depth to your finish as well.
Types of polyurethane
When choosing a polyurethane, always choose the type that matches your stain. If you used a water-based stain, use a water-based polyurethane. Oil-based stain? Oil-based poly.
(Personally, I’m not a fan of water-based stains and polyurethanes. Sure you save on cleanup effort somewhat, but water raises the wood grain. You’ll need to knock the grain down with a sanding.)
You’ll have some choices of sheen. Gloss, semi-gloss, or satin. I like semi-gloss for trim because it offers good durability and cleanability and adds some depth to the overall finish. Gloss can offer some extra pop in your finish, but for some, that might be overdoing it. Satin gives a bit of a matte finish but it’s a bit less durable. Scuffs and dings tend to be more noticeable with a satin finish. It wouldn’t be my first choice for baseboards.
Prep work
If I’m happy with my final stain color, I’ll try not to sand my piece unless I have to. Any sanding at this point risks removing some of the color.
Before I begin brushing on my poly, I’ll hit the wood with my shop vacuum. If your project has been sitting in the basement or garage for a day or two, always vacuum it to grab all the little dust particles that have invariably fallen on it.
Nothing wrecks a nice smooth polyurethane finish like dust.
After vacuuming, wipe it down with a tack rag.
Now, if you’re using an oil-based polyurethane, use a natural bristle brush. For water-based, use a synthetic brush.
Crack open your can of poly. DO NOT shake it before opening it. Shaking it will introduce lots of air bubbles which could ultimately mar your finish. Rather, stir the polyurethane thoroughly, from top to bottom, using a paint stick.
If you’ll be applying polyurethane to numerous pieces at once, try and go back and give the can a stir every so often.
Application
I like to work straight out of the can. I’ll dip my brush in about a quarter of the way and then wipe off the excess on the inside edge of the can.
Then, slowly and methodically, I’ll brush on the polyurethane with the grain. Use long, drawn-out strokes. Keep a wet edge and look to achieve a flat smooth gloss. (Even if you’re applying satin, it will look glossy when you lay it down.) Keep the air bubbles to an absolute minimum. The small ones will take care of themselves as the poly dries.
Your first coat should be thin. Lay it on too thick and you’ll have drips all over the place.
Let the piece dry until it no longer feels tacky or sticky. That could be in as little as four hours or as long as overnight. Make sure you’ve got good ventilation if you’re using the oil-based poly. This stuff gives off some serious fumes.
Once it’s dry it probably looks decent but we’re not done yet. Give the piece a light sanding with a fine grit sanding sponge or 180+ grit sandpaper. Don’t go nuts though, you don’t want to sand through the first coat. Run your fingers over the finish, does it feel smooth? As soon as it feels like glass, stop.
If you have any drips, you can shave them down first with a razor blade. Then sand them flat.
Fire up the shop vac and then wipe it down with a tack rag. Brush on the second coat and let it dry.
When you come back the next day, the finish will be as smooth as glass. If you need extra protection (baseboard trim for a mudroom, for example) you can apply a third coat. Again, sand using a fine grit sponge or sandpaper and re-coat.
I hope you’ve found this guide to applying polyurethane useful. I focused on pine wood as it’s a common wood type for trim and other small projects, but it applies to all types of wood — hardwoods and softwoods.