Finishing an Exposed Exterior Door

The harsh conditions of a south-facing exterior environment can trash any finish, and the door under it. But here's a closer look at the best bets. October 3, 2011

Question
I've used the ICA finishes before on bar tops but I was thinking of using the ML Campbell equivalent on an exterior door. What will happen to the relatively thick finish around where the raised panel profile meets the sticking when the panel expands and contracts? Will it crack, thus making is susceptible to failure? The door is a 6 panel, 48x96. The panels will be quite wide, approximately 15".

Forum Responses
(Architectural Woodworking Forum)
From contributor J:
I really think two-part polyurethanes are far more trouble than they're worth. First of all, the cans themselves can easily cost ten times as much as the regular stuff (and you better buy twice as much as you think you're going to need). The life-span is very dependent on the number of applied coatings.

A two-part finish will indeed last about three times longer but you'll still need to re-coat about every year or so. If you do happen to "let it go", you'll have an extremely tough mess to remove (every square-inch with a heat gun, scraper and/or very coarse sanding). I believe the most flexible clear coating is still "marine spar varnish" and I would certainly go with that or perhaps Cetol.

Actually I am becoming a firm believer in Cetol. The newer formulas look great and last just as long as any two-part polyurethanes. Best of all, the yearly re-coat should never involve any heavy sanding or stripping of any kind (ever again). If you do "let it go" it eventually just flakes off with a brush.

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From contributor O:
Sikkens Cetol is the product. If you consider the fact that all exterior wood finishes eventually fail, then the problem becomes how to refinish without difficulty. This is where Sikkens excels, depending upon exposure of course. Nothing will work on full South, no protection for long. All the super hard, bullet proof finishes look great, but when they fail (and they will) they are near impossible to refinish. It’s far better to go with something removable and user friendly.

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From contributor N:
It really depends on the exposure. I tried two part poly on my shop doors that get full exposure to rain and sun and the finish didn't even hold up for a full year. After constant recoating I finally stripped them (very difficult) and finished them with spar varnish and they are holding up much better.

I do have a door finished with two part poly that doesn't get much weather and it is holding up much better, but spar varnish probably would have been a better choice for that one also.

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From contributor G:
I use Target EMTECH 9300 for exterior doors. Waterborne polycarbonate urethane that is very flexible.

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From contributor O:
I recently recorded a surface temperature of 165 degrees on a finished and installed mahogany door that is south facing and in the sun all day - air temp 50 degrees, overnight about 30. The temp would be about 15 degrees higher in mid-summer. The customer finished with spar varnish about a year ago, and the finish is cracking at joints due to wood movement. The spar is separating from the stain underneath, and will soon allow water under the varnish and the failure will increase to where removal and complete refinish is the only option.

This exposure is not a warrantable situation for a door maker or finisher, but people still do it all the time. Neither the wood nor the finish can stand the expansion and contraction of the extremes. Some door designs will fare better, but all are at risk.