This article was inspired by a question from Jeff. He writes:
“I am using Helmsman Spar Urethane to refinish the front door of our house. It’s a one piece flat door — no recessed panels. I am applying it with a quality brush but I still get brush marks and hi-low points, also tried a sponge brush with same results. I know it’s barely noticeable but I absolutely abhor brush marks. After 4 coats, I have managed to sand it down fairly flat. Is it possible to thin Helmsman and spray it with an HVLP gun? There is a Helmsman spray can available but I have so much Urethane left over. To thin or not to thin?”
Polyurethane, whether it’s an outdoor or indoor formula, takes a little finesse to apply without bubbles or brush strokes. You need to use the right brush and it takes a little practice to get the technique right. Sponge brushes are certainly another option, but the results are not always a significant improvement. Most of the trouble comes from the fact that the urethane solution is so thick. It can be like trying to lay down an even coat of corn syrup. So one way to significantly improve the workability of your urethane is to thin it with mineral spirits. Thinning by about 10-20% will make brushing significantly easier. And thinning even further would allow you to spray via HVLP, but that’s definitely not my favorite way to apply an oil-based varnish. Thinning allows the finish to self-level a little better, and ultimately that will help you avoid brush marks. The only disadvantage is you need to apply more coats to get the desired film thickness you are after. But if the payoff is a smooth finish with no brush strokes, who cares, right?
Now there is yet another way to apply polyurethane that I feel is the easiest, beginner-friendly method. And frankly, it’s the method I still use. It involves abandoning brushes all together. Here’s what you do. Thin the varnish 50% with mineral spirits, paint thinner, or naptha. Then get yourself a clean cotton rag (old t-shirt material works great), and fold it into a nice square applicator pad. I like it to be palm-sized. Wet the applicator with your thinner and then dip it into your diluted finish. Wipe on smooth thin coats, overlapping the previous stroke with each pass by about half. This method will result in a super smooth finish with no bubbles and no brush strokes. But remember, you will need to apply twice as many coats to get to your desired film thickness. Because you are actually applying less material to the surface, the dry time is significantly decreased as well. So in some climates (hot and dry), you’ll be able to apply as many as three coats per day. Just remember that when you use a wipe-on urethane, you aren’t trying to flooding on a real thick layer. You are just coating the surface evenly with a thin wet film.
If you want to learn more about my particular finishing method, you should check out my video A Simple Varnish Finish. My goal with the video was to demystify finishing by going over each and every step of the wiping varnish process. Even someone new to the world of finishing can create a show-stopping finish if they follow the methods outlined in the video. So check it out! And for those that are wondering what my preferred outdoor varnish is, its Epifanes Marine Varnish.