I often get asked, “What can I put on wood that will protect it outside?” My follow-up question is, “Do you want a finish that builds up to a film or just something that soaks in, like an oil?” If the answer is a film finish, I recommend Cetol Door & Window, from the Sikkens Proluxe line of wood finishes. It holds up better than any other outdoor finish that I have used, and every painter that I know and trust uses it too. I have found Cetol to last almost twice as long as the next class of film finishes.
In my teenage years, I tried regular ol’ Minwax polyurethane on the wood bed of my restored ’63 Chevy pickup truck, and I couldn’t believe how fast it started to peel. One St. Louis summer of constant heat and sun made it look like it had a bad sunburn with lots of dead skin.
After that, I moved on to other products like spar urethanes (Minwax also makes one of those, called Helmsman’s). Yes, it lasted longer, but not LONG. It started to look bad after a little more than a year. It didn’t totally fail all at once, but enough areas were falling apart that it didn’t really matter – it just looked bad.
In defense of these two products, the bed of a pickup truck is a tough environment. It gets direct sun, extra heat with no breeze, and the surface is horizontal, so water has a much better chance of nosing its way under the finish. It really gets no more demanding than this for a wood finish.
With yearly maintenance, the spar urethane could be kept looking reasonably good, but eventually the maintenance gave way to submission and the weather won. The boards still looked alright (nothing rotted through), but there were always spots where the finish failed and the beautiful clear-coated lumber had cracked finish and gray spots of raw wood.
It wasn’t until much later that I was introduced to the Sikkens brand name from a friend in the St. Louis Woodworkers Guild. He had great things to say about their products, and then I started noticing them being used by different painters on different jobs around town – and consistently. Nobody was using anything else, at least not anyone that I trusted, so I started using them.
Originally, Cetol only came in various forms with added stain. Some of the colors were pretty light, but none were clear. I believe the color is added to help with UV protection, but it does nothing to help color matching or achieving a clear finish. Now, with the name change to Proluxe, a colorless version is available, and it is the one that I use.
The can says that Cetol should be applied with a brush and not rolled or sprayed. I haven’t rolled it, but I have brushed and sprayed it, and both worked fine. Spraying is more difficult, and probably not recommended by the company, because the Cetol has a long working time and tends to sag if applied too liberally. That same working time is great for brushing and allows large areas to be worked and reworked to blend brush strokes. If you do try to spray it, start with a light application and allow it to settle for a second so you can get a feel for how it is going to sit down. Keeping a wet edge isn’t critical because it doesn’t even start to get tacky for a long time and the following passes will easily blend together. If you do spray the Cetol be vigilant about finding areas that start to sag or run and simply blend them in with a brush.
Cetol, like many other finishes, takes three coats to build a good protective barrier. A fourth coat will add a bit of extra protection, but isn’t required. I would opt to skip the fourth coat on the initial application and put that energy to a maintenance coat later down the road.
Regular maintenance is critical to keep the finish from failing. Keep an eye out for areas that start to crack and get another coat on as soon as they appear. If you wait too long it will be necessary to completely strip the finish and start over.
Cetol should last for several years without maintenance on vertical surfaces and even more if they are protected from the sun. Horizontal surfaces in the sun will probably last a maximum of two years before they require attention. Both of these time-span numbers are not great, but they are at least twice as long as the spar urethanes. If you jump around the internet and read about other choices or recommendations from other woodworkers, you won’t find anything that lasts longer.
Unfortunately, three years is really the maximum for an outdoor clear finish with sun exposure. Of course, if you know of a finish that lasts longer please let all of us know about it. But, do watch out because the world will beat a path to your (long-lasting, clear-coated wood) door.
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