Finish Choices for a Mahogany Exterior Door

The perennial problem: a clear finish tough enough for outdoor exposure. Bottom line, there's no such thing: sooner or later, the door will need recoating. Here, pros consider the tradeoff between the durability of the initial application and the difficulty of the eventual refinish job. June 28, 2006

We are a millwork company that does mostly interior work. We are getting ready to build an exterior door out of 8/4 Honduras mahogany and I am interested in what others think would be the best clear finish for the exterior of the door. The interior will be primed and painted. Right now, our plan is to pre-seal the door components with West System epoxy before assembly, and use an exterior varnish after assembly.

Forum Responses
(Finishing Forum)
From contributor F:
Regardless of what finish you use (I'd go with a two part poly, but varnish will work) this door is going to require regular maintenance, and likely the occasional refinish. This is especially true if the door faces south (sun) or is exposed to seasonal temperature variations. Your customer should be made aware of this so that it doesn't become your job.

From contributor U:
I would suggest using Sikkens Cetol, as this would allow the customer an easy maintenance task.

From contributor S:
After 30 years, I found Sikkens is the only finish I will recommend nearly every time. We pre-finish a third of our exterior doors with Sikkens and everybody is happy. Most every door on our prototype website is finished with Sikkens. Polys and catalyzed will crack and peel under UV, and are then a real problem to remove and recoat.

From contributor P:
I have an exterior door job coming up as well. Can you spray Sikkens Cetol? What is the thinner? This client wants a glazed finish to make the door look a little aged. Is this possible with Sikkens?

From contributor R:
To contributor S: How many years does the finish last in moderate sunlight? How many coats are you putting on, and does the film get thick enough to prevent water from getting between the panels?

From contributor S:
Sikkens can be sprayed - check with their technical people for do's and dont's. Sikkens will last anywhere from 9 months to 8 years here in Indiana. The short side is South facing, full exposure. I recommend paint for these types of exposures, but people want what they want. The longer periods are for north facing, with overhangs.

Sikkens goes on as 3 coats, and has a moderate thickness. As it ages, it gets thin. It will actually look as if it is worn off on the high spots after a while. This is when it is time to recoat. It will crack or peel if there is too much finish on the surface - it is designed to fail the way it does to facilitate recoats.

From contributor R:
Dave you stated in your e-mail that you do all kinds of pre-finishing of panels before they are set into the frames. But as finishers we get a raw door. When I tested Sikkens cetol on a frame and panel door it lasted 6 months. This was on the roof of my building, a tough test because of exposure but still I didn’t think that was very good.

From contributor S:
The roof test is merciless. If you haven't already, test spar varnish, epoxy, urethane, polyurethane, conversion varnishes and anything else you like alongside the Sikkens. I bet lunch they will all fail in about the same length of time.

The difference is the way they fail. Sikkens is easy to clean up and recoat - even recolor. Homeowners can do it on a Saturday afternoon. The spar varnishes, urethanes, polyurethanes and especially conversions all fail with cracking and peeling, making a strip and total refinish job costly and frequent. One guy around here gets up to $5,000 to redo a failed conversion varnish entry, exterior only! It is hard to prepare an owner for that kind of expense.

I am the first to say that a stain/clear finish on an exterior wood door will require periodic maintenance just like a wood boat. People want what they want. Some will tire of the maintenance and paint, and I don't blame them. But at the point of sale, they don't really consider the downside as I explain it, they just want that beautiful door. And in my opinion, Sikkens is the best way to give it to them.

By the way, 3 of 4 Mahogany doors on my house are painted on the exterior; the odd one is behind a storm/screen door. The entrance door to my shop is a burned off oil finish, leaving the wood exposed for 7 years, as an example of what not to "try at home." The door hasn't moved a fraction, and hasn't leaked a bit, either. Looks pretty bad, but is as solid as the day it was built.

From contributor S:
What happens when the ML Campbell eventually does fail? How is it removed? If it guaranteed 50 years, then I'm sold. If not, I’ll stick to Sikkens and the big picture. All wood finishes fail in exterior use - it is how they fail and then recoat that makes the difference.

From contributor U:
Contributor S has it completely right. It's fine having a finish that lasts 10 years but if you then have to set off a thermonuclear device to prep it for re-coat, what's the point? Exterior wood stains such as Cetol are designed to erode, thus indicating when they need maintenance. Then you rub down with ScotchBrite, touch in any bare areas with Cetol 1, and bring forward with Cetol 23 and a full coat of 23 over the lot.

The secret to durability with any wood stain is the film build. We find that many people apply it sparingly and it looks good, but only does 12 months on window sills and bottom beads. If the Cetol 1 is applied to the point where it is starting to run, left for 5-10 minutes then re-distributed with a wiped out brush, this gives the best basecoat. The 23 is then applied in two full, flowing coats to achieve a wet film of 60 microns each coat.

From contributor B:
I have a Honduras mahogany exterior door with southern exposure. The poly is peeling badly after only one year. What is the best way to remove it (do I need to take it down to the bare wood?) and what is the best way of putting a new finish back on? Are there multiple Sikkens products I need to apply?

From contributor U:
The best way to remove poly is with a methylene chloride stripper, followed by sanding to remove the greyed surface fibers. Cetol is a two product system on joinery - a first coat of Cetol1, followed by two coats of Cetol23 or Cetol TGL. Failure to completely remove the varnish will result in a patchy finish as the varnish prevents the Cetol1 from being absorbed.