How to apply polyurethane without brush marks

Polyurethane is a plastic material that exists in various forms, though for today’s list, we’ll be talking about it in its varnish form. A nice coat of polyurethane varnish brings out wood’s natural beauty and grain. On top of that, when brushed on other surfaces like metal or layers of paint, polyurethane forms a seal, protecting the surface from damage and giving a nice sheen.

Unfortunately, using polyurethane can often leave behind brush marks that ruin the overall look of the surface you’re working on. Sometimes, if the varnish is the right consistency and applied properly, it can resolve itself, but often times these marks make their way to the final product. Even when using high quality expensive brushes, you can still leave marks at times. So, how do we avoid this?



Well, worry not DIY fans, today we’ll be breaking down some of the ways you can avoid this from happening in the first place. All you’ll need are some good tools, sometime and the patience to get it done, and you’ll be enjoying your perfectly smooth surfaces soon enough.

What you will need for this project

Medium sandpaper (100-grit), fine sandpaper (150-grit), extra-fine sandpaper (220-grit), 600-grit sandpaper, spray bottle filled with water, foam paintbrush, lamb’s wool applicator, mineral spirits like paint thinner, one can of oil or water based polyurethane, bucket or container for mixing, shop vacuum and razor blade.

Step 1: sanding the surface

To begin, you’ll want to take your surface and sand it with progressively finer grits of sandpaper. Paper with a higher grit number removes the deeper scratches left by paper with a lower-numbered grit, so slowly build up from medium sandpaper to fine sandpaper to finally extra-fine 220-grit sandpaper in order to get the best finish possible.

Step 2: spraying the surface with water

Next, if you’re sanding wood, you’ll want to get a spray bottle filled with water and just gingerly spray the surface with it. Spraying wooden surfaces with water after sanding is an integral step. It helps raise the wood grain up, allowing you to properly sand things out after. Skipping this step will only lead to issues later on. If you’re not sanding wood however, you can skip this and the next step.

Step 3: sand the surface again

Once again, you’ll want to sand the surfaces back down to where they were, slowly progressing from medium to extra-fine. Now that you’ve done this, the grain won’t raise again when you’re applying polyurethane on it, allowing for a nice even coat with no brush marks.

Step 4: remove leftover dust

When the wood is free of any unevenness or blemishes, you can proceed with removing any of the leftover dust. Using the shop vacuum with a soft brush attachment can do this fast and efficiently, though if you don’t have one you can instead opt to just sweep and wipe it down until everything has been effectively dusted.

Step 5: properly mix your polyurethane before use

This one’s common sense, but many people can miss this crucial step in their excitement. Stir up your polyurethane before using it so that it can mix with the sediments inside and actually be used. Make sure you don’t miss this one or it’ll be a waste of a good project.

Step 6: pour out polyurethane into bucket or container to make sealer

Pour out some polyurethane into your container and then mix in some of your mineral spirit to thin it down. This is done to make an effective sealer for your surface. It’s not going to look good, but the purpose of this is to ensure that all the stains are sealed. Some stains are self-sealing however, so make sure to refer to you can for this information.

Step 7: apply sealer to surface

Now, you’ll want to apply a nice coating of sealer to the surface of your project. Gently brush it on using your foam paintbrush, though be careful not to go too fast lest you develop air bubbles on the surface. Brush with long, even strokes and try to smooth out any drips into the surface. Keep this up until the surface is fully coated.

Step 8: leave to dry overnight

Let your project dry overnight. It’s important that you don’t leave it to dry somewhere too hot, or the polyurethane won’t have enough time to resolve any lesser brush marks on it. This can lead to a botched project, so keep it in mind.

Step 9: apply polyurethane from can

We’re finally ready to really get into things! Get your lamb’s wool applicator and gently dip into your mixed can of polyurethane. Then, apply it to the surface. The general rules that came with applying the sealer also apply here, so use longer strokes and get every corner slowly. Avoid dragging your applicator back and forth, as this can form air bubbles. Then, allow to dry overnight.

Step 10: shave off any bumps or drippings

Self-explanatory, use a razor blade to shave off any bumps or drippings from the dried surface. This step is important, but you can’t miss it. Be careful when doing this, try not to dig into the finish itself.

Step 11: sand the dried surface

After all that, sand down the dried surface using extra-fine sandpaper, until everything has been smoothened out.

Step 12: repeat steps 9 to 11 until desired smoothness is achieved

From here, you’re basically done and just getting a final finish that you’re happy with. It usually takes two to three cycles, but once you’re happy with what you have, you can move on to the final step.

Step 13: use 600-grit sandpaper for final sanding

Now that it’s looking good, use 600-grit sandpaper and give that surface a sleek, smooth finish that you can be proud of. With that, you should have a perfectly polyurethane covered surface with no brush marks or air bubbles anywhere!

Conclusion

We hope this step by step guide on the proper way to apply polyurethane without any brush marks has helped you achieve that immaculate shine you want. It’s an impressive effect, and even though it takes a lot of time and effort, we’re sure the final results will be well worth it.