Foam Brush Polyurethane Finish on Walnut

When I was just starting out, I joined the “Poly-everything” crowd. It was a long time ago. It seemed to me that polyurethane was all anyone was using for a clear topcoat. I loved the durability. I still do for projects where polyurethane-level protection is warranted. Unfortunately, poly is a pain. For one thing, it dries SO SLOWLY! For another, it doesn’t play well with others, or even with itself. For yet another, repairs are difficult. But if you need the protection, there is nothing better.

Polyurethane’s downsides are on full display if you are seeking a glassy surface on open-grain wood. I made three walnut doors for our master bedroom. They are sliders (not swingers), so structurally speaking, I was free to make them light. I went with a wink toward shoji screens with rice paper panels that appear to be suspended in their walnut frames. I wanted a pretty formal looking frame around a more organic, natural looking screen. To me, that meant a glassy finish on the frame with barely-finished screen dividers. This article is about getting a hard-as-nails, glass-like finish on walnut. I’ll cover some of the tricks I picked up while finishing the frames, including the use of foam brushes.

Finishing options to get the result I wanted (of which I am capable) are spray lacquer, French polish, or polyurethane. Full disclosure, this is the only project I’ve ever started in my shop that I set aside for any length of time before completing. This door is for my wife’s bathroom. When the old swinging door came down during a remodel, she made this crazy demand that I produce a new door for her bathroom reasonably quickly. I made all the doors at same time but only finished and hung the one on Her Majesty’s bathroom. The other two are for closets, which don’t actually need doors at all. So I set my two unfinished doors aside. Eight years passed.

Why the backstory? Because 8 years ago I was not able to spray lacquer and I didn’t know how to apply French polish. I finished that first bathroom door in polyurethane. All three doors will hang virtually next to one another. So now, like it or not, I have to do the other two the same as the first.

This process is not original, but here’s how I did it. First, I waited until I had several weeks when there were no other projects happening or planned for the shop. This is a big reason for the 8-year time lag. This is critical, because once all the prep is complete and you’re ready to apply a coat of finish, the routine becomes a once-a-day affair. A quick scuff-sanding, let the dust settle, a coat of poly, and walk away. And you can’t do anything else in the shop because you want to work as cleanly as possible. One of poly’s greatest challenges is its lust for dust. It takes a lot of patience and discipline to deny it one of its favorite things.

I considered setting up the shop so I could finish both sides of both doors at once. Perhaps a flippy-type thing where the doors were suspended between a couple step ladders so I could walk around them and flip them over and keep on working. That would have taken up a huge amount of shop space, introduced more dust management issues, sag management issues, and more risk to the finish I’d just applied during the flip. I decided instead to embrace this as an exercise in patience and finish one side of each door at a time. Turns out, each door took about 8 days per side, multiplied by 4 sides, so about a month altogether, typically working about 30 minutes per day every day (plus time for dust to settle). Patience, indeed.

I’m a fan of foam brushes for brushing varnishes. Thomas R. Shrunk wrote a great article on how to apply fine finishes with foam brushes (FWW#116, pg.116). Being a supreme hater of cleaning brushes with solvents, I was an immediate disciple. With a little experience, I have found the lowly foam brush equal to any other brush I own for a finish like this. I would much rather waste a stack of single-use brushes than mess around with solvents every day for a month.

In broad strokes, the process is to finish sand the project to at least 220, then fill the grain. Sand and reapply grain filler as necessary to fill the wood pores as well as you can. Apply coats of poly and sand them down until you establish a perfectly level surface. Apply one perfect show-coat. Rub it out with steel wool and wax.

As with any finish, prep is key. I’ve never regretted spending too much time prepping for a finish, but I’ve experienced lots of regret from prepping too little. Sanding is not fun to me, but it is very efficient compared to stripping finish. Spend the time to sand to perfection. Spend regret on something else.

Cleaning the environment is an important part of the prep process for a finish like polyurethane because it stays wet so long. Clean the shop. Keep it closed up if you’re finishing during the flying-critter months. If fumes are an issue, it is easier to wear a respirator in a closed space than it is to sand out a bug in an open one. The less activity there is in the shop, the less dust will mysteriously appear in your varnish.

Grain filling is messy and tedious. You can skip this step and simply apply more coats of poly until you build up a smooth base, but grain fillers will speed the process. Behlen makes a good grain filler. I was recently turned onto Aquacoat and there are things about it I like and things I don’t. I hope soon to try old-fashioned Plaster of Paris and write an article just on filling grain. Regardless the product, I’ve learned that getting a fully filled surface for top coating isn’t easy. I’ve accepted that all I can expect from Behlen and Aquacoat is to get a nearly level surface that can be leveled by building up the top coat. Maybe you’re better at filling grain. Who knows? My advice is to do your best and move on. Two passes with grain filler is about all I have patience for. Aquacoat is hard as stone, making sanding a bitch.

These photos show me troweling on Aquacoat with a credit card at an angle to the grain. After about half an hour, I follow the first coat up with a ragged-on second coat 90 degrees to the grain. When the grain filler is fully cured, I sand it down to the most level surface I can get at this point.

One tip I’ll share is to resist the temptation to level your grain filler with a random orbital sander. It is simply too easy to grind through the filler and re-open the grain. Hand sanding is real work on a product as hard as Aquacoat but sanding through the filler is defeatist.

With grain filled and everything re-sanded, I apply my first coat of poly. The urethane products I use are the venerable and ubiquitous Minwax Fast-drying (hahaha) Polyurethane and Minwax Wipe-on Poly (both in clear satin). I follow Shrunk’s advice, laying down “puddles” of product to be absorbed and distributed by the foam brush as it passes over the surface. Using overlapping strokes, I make one single pass at a very slow speed- I’d say about 1 second per inch or a tad slower- trying my best to keep the brush moving smoothly. Visualize the foam drawing in and letting out the varnish as it passes over the puddles and you’ll get a feel for how slow to go. Try not to move in a jittery stop-and-go fashion. When advancing from a finished area to an unfinished area, come down on the work into the wet section with a gentle “airplane landing” technique.

For horizontal surfaces, I lay it on thick during the building stage. For vertical surfaces that cannot be rendered horizontal, the process is the same but with much less product to avoid sagging. Less product equals more coats, which equals more days… you get the picture.

The next step is to walk away. Secure the space to reduce dust being kicked up. Plan ahead- my shop is also my garage. I back cars out before I apply a coat of finish if I know we’re going somewhere. I don’t want the garage door going up and down over wet varnish.

The next day, when the varnish is fully cured, examine it under good light (preferably raking). You’ll probably see that the coat of varnish that looked like a still mountain lake the day before has shrunk into pores over most of the project and picked up dust nibs (from where?!?). That’s a normal part of the building process. Give the whole project a light scuff-sanding with 220.

I like to vacuum my project and then walk away for a couple hours to let any dust I cast into the air settle. When I return, I wipe the surface down with my hand, cleaning my hand on my shirt often. The oils on my hands do a great job of picking up dust. After several passes, and when I can’t feel anything foreign on the project, I lay down another heavy coat of varnish, pitch the brush and walk away. Being disposable makes the foam brushes really shine.

The next day you’ll probably see more shrinking, but less than yesterday. Depending on how nearly-level your surface is, you may want to see whether two heavy coats is enough build to sand them down to a perfect surface. I use my random orbital sander with 220 for this, paying close attention to how much sanding it takes to get a perfectly even scratch pattern. Shiny low spots stand out like Bic lighters at a Dead concert. If you’re sanding more than just a little to get rid of the shiny specs, abort. Scuff the project and apply another build coat. Protect yourself from sanding through to raw, open-pored wood.

You’re done when you’ve built up enough varnish to sand it to a perfectly even surface, free of shiny spots and without getting into the wood. At this point, I like to ramp up the grit size. From 220, I might jump to 400, then 600, then 1200 or even higher.

The finer grits help with my final application- a heavy, brushed-on coat of Wipe-on Poly applied in exactly the same way. Wipe-on Poly lays down faster because it contains a lot more solvent. It also dries faster because of the higher solvent content. In a clean shop, I can brush on a coat of wiping varnish and return the next day to a surface ready for 0000 steel wool and wax.

If the final coat isn’t ready for waxing due to some defect or bug, scuff it again and brush on another coat of wiping varnish. The waxing step is more for feel than appearance, so you want the appearance to be pretty much perfect. You can’t apply another coat of poly over wax, so waxing is a one-way gate. Be sure you’re ready to pass through.

This is not a hard finish to apply. It is a tedious finish to apply. It is an exercise in patience, and it renders the shop useless from start to finish (pun!). If you have the patience and you want the appearance and durability of glass, the results are undeniably spectacular. I sincerely hope you don’t wait 8 years from the time you build your project until the time you apply the finish but if it takes that long to find a month to apply this finish, you’ll be forgiven.