How to treat an exposed wooden door

June 9, 2018 8:49 AM

Woodworkers of Metafilter! My new house has a ton of beautiful wood that appears to have been installed by an amateur who didn't like finish sanding or putting a second coat of polyurethane on anything. This is mildly irritating in most instances, but the south-facing solid wood exterior door is in rough shape, and I need to figure out the best way to handle it.

The door appears very nearly untreated, and almost thirty years of sun exposure has left the grain standing out very strongly and there are some small cracks starting. It's a beautiful door, and I'd rather not replace it - will some sanding and my standard Minwax polyurethane be my best bet, or is there a better option? It does get exposed to moisture, but there's a solid storm door installed as well, so it doesn't get soaked in rain or anything.

I am very much an amateur, and any wisdom is welcome!

posted by restless_nomad to Home & Garden (15 answers total)

Sanding plus polyurethane will probably last just fine for several years.

I think boat builders coat wood in several layers of epoxy and then paint several layers of exterior varnish on, the epoxy seals the wood and the varnish protects from UV. But that seems like overkill for a door.
posted by gregr at 9:07 AM on June 9, 2018

Make sure you get the edges of the door. You'll probably want to take it off its hinges to finish it. If the previous person didn't do, say, the bottom of the door, that could be a mess later.
posted by amtho at 9:13 AM on June 9, 2018

I think if the door has spent 30 years there and only has minor weathering, then a curing oil finish (like pure tung or teak oil) is an ideal choice because it will make the wood look beautiful, is easy to apply and probably will last many years before needing to be (relatively easily) redone. If you use a hard film finish like epoxy + poly, it may last a lot longer before needing to be redone but when you do it will be more of a hassle (all the old stuff must be removed) and in the meantime will not look as nice.
posted by Poldo at 12:09 PM on June 9, 2018

Spar varnish.

You probably need to take it off to do the sanding well on all five exposed faces. Is it standard enough that you get get a janky replacement at a used-building-supply place? (Or heck, amybe you live somewhere you can just use the storm for a week in summer.)
posted by clew at 12:12 PM on June 9, 2018

I agree with Poldo that your door has shown itself to be a worthy survivor -- and I'd probably find it more beautiful as it is now (people are going to extreme lengths to get a raised grain effect on their wood these days, for example) than after anything you're likely to do to it -- but tung oil is a powerful irritant and sensitizer for many people, including me, and may exacerbate any pre-existing inflammatory condition you may have, or even kick off some new ones. And I believe tung oil is a principal ingredient of teak oil, too.

So I would recommend boiled linseed oil instead.
posted by jamjam at 12:41 PM on June 9, 2018

This door is not exposed to weather, though, right?

I like an oil finish, myself, either way, but the work time is a lot more than Spar varnish. (Formerly for varnishing spars, on ships.)
posted by Lesser Shrew at 2:45 PM on June 9, 2018

I tend to agree on the use of an oil, however note that teak oil is an oil and varnish mix and not generally specified for outdoor use, I wouldn't expect the varnish side of things to last very well in this instance.

I have not heard of tung oil being an irritant and I have fairly intensively researched this. Tung oil is often (perhaps usually where you are) sold as a "tung oil based" finish with many other ingredients included, some of which will be irritants in some cases. So if you are heading down this route read the label carefully and dig out the product's MSDS online if you are worried.

Boiled linseed oil is not especially durable and will generally produce quite a dull finish. It's easy to apply though and very easy to repair.
posted by deadwax at 4:21 PM on June 9, 2018

Here's what ikkyu2 had to say about tung oil:

Tung oil is natural but it's hardly non-toxic. It's a potent irritant.

A very blasé about toxics woodcarver of my acquaintance told me a few years ago that the scuttlebutt among woodcarvers is that tung oil is associated with Parkinson's.
posted by jamjam at 4:38 PM on June 9, 2018

like mefi cat questions, i don't see how we can possibly answer this without pics.
posted by at at 5:49 PM on June 9, 2018

Wood exposed to the elements is going to need regular upkeep. The simplest and lowest hassle thing to do is pull it down, investigate it thoroughly for any rot (dig out and patch if so), then gently sand it to break down any standing fiber, then scrupulously follow the manufacture's instructions for an exterior rated polyurethane, e.g. Minwax Spar Urethane. You'll need to revisit the door every n years where n depends on exposure to sun and rain, but if you keep it up it'll last forever.
posted by seanmpuckett at 6:55 PM on June 9, 2018

Yes, I was responding to ikkyu2.

I can't find any evidence that pure tung oil is a significant irritant, googling tung oil and Parkinson's returns exactly zero relevant results. I have been published on wood finishing and not come across any issues like this.

I am all for acknowledging the hazards that products present, but in the interests of people like the OP getting sensible answers I prefer to use verifiable data. The MSDS for tung oil is quite tame, after reading it I'd suggest wearing gloves when applying it and that's it. It is zero VOC. The thinner or dryer chosen is going to have far more bearing on any health effects. (Gum turpentine works well)

More on topic, I also agree that an external varnish would work very well, it's a lot more work to get on nicely is all, and requires a good deal more skill.
posted by deadwax at 8:16 PM on June 9, 2018

Here's how my Merck Index (11th Ed. P.1673) describes tung oil:

Pale yellow liquid; characteristic disagreeable odor.

If tung oil does indeed have zero Volatile Organic Compounds, what do you suppose people are smelling?

Tung oil is extracted from the seed of a tree in the genus Euphorbia, commonly called spurges. Here's what the Euphorbia article on Wikipedia has to say about the irritant properties of Euphorbias:

Irritants

The milky sap of spurges (called "latex") evolved as a deterrent to herbivores. It is white, and transparent when dry, except in E. abdelkuri, where it is yellow. The pressurized sap seeps from the slightest wound and congeals after a few minutes in air. The skin-irritating and caustic effects are largely caused by varying amounts of diterpenes. Triterpenes such as betulin and corresponding esters are other major components of the latex.[14] In contact with mucous membranes (eyes, nose, mouth), the latex can produce extremely painful inflammation. Therefore, spurges should be handled with caution and kept away from children and pets. Latex on skin should be washed off immediately and thoroughly. Congealed latex is insoluble in water, but can be removed with an emulsifier such as milk or soap. A physician should be consulted if inflammation occurs, as severe eye damage including permanent blindness may result from exposure to the sap.[15] When large succulent spurges in a greenhouse are cut, vapours can cause irritation to the eyes and throat several metres away. Precautions, including sufficient ventilation, are required.

Oil from the seed is obviously not the same as sap, but my guess is that the "disagreeable odor" of the oil arises from some of the same terpenes that make the latex so toxic, but at much, much lower concentration than in the latex.

Most people are evidently not sensitive to the irritants in tung oil at that low level, but I am, and I don't think I'm unique.
posted by jamjam at 10:47 PM on June 9, 2018

if you are concerned about toxicity then pure linseed oil would be better than "boiled". pure oils take a relatively long time to polymerize which is why the boiled variations have chemical dryers put in to speed up the process. the pure tung oil I buy from woodcraft does not have accelerator additives.
posted by Poldo at 7:38 AM on June 10, 2018

like mefi cat questions, i don't see how we can possibly answer this without pics.

Fair enough! I took a couple: the door, a closeup of the grain, and a closeup of the cracks starting.

This door is not exposed to weather, though, right?

There's a storm door between it and the world, although we usually just leave the screen open in the summer.

This is exactly the range of ideas I was hoping for, thanks everyone! Further thoughts very much appreciated. I can totally get a cheapo temporary door at the Habitat for Humanity store if I need to (or just leave the storm up - the main part of the house isn't really temperature-controlled in the summer.) And it occurred to me to check the laundry room door (west-facing) and it will need the same treatment, whatever it is, although it's not quite so badly weathered as to be cracking yet.
posted by restless_nomad at 8:52 AM on June 10, 2018

I have a beach house with doors made of solid old-growth pine. Very tight grain and beautiful patterns. Previously, they had been painted, which was a shame. However, the paint did a good job of fighting UV rays reflecting off of the water, and general weathering. The polyurethane that was used to coat them had failed after 10 years. I took them down and sanded as much as I could, then applied three coats of Spar Varnish. They still look good.

One thing that you need to take into account for this project is that the door has to come down to be worked on. This is no project to do in place. Also, this sort of thing takes a lot of prep time. You should decide if you need a temporary door in place during the job. The other thing is that the finish may continue to smell for a week or so after it is cured. It is best to allow for that time period as part of the project.
posted by Midnight Skulker at 9:47 AM on June 11, 2018