How to finish the wood in a sauna?
June 28, 2020 9:04 AM   Subscribe

I want to treat the ash wood benches but what can I use that won't smell?

We are on the cusp of finishing a wood heated sauna and have benches made of ash. I want to finish them with an oil that doesn't impart a smell when the sauna is heated.
posted by InkaLomax to Health & Fitness (13 answers total) 3 users marked this as a favorite
 
I don't know if ash wood is any special, but where I come from (torne river valley in northern Sweden/Finland) and where saunas are everywhere, we typically don't use any finishing such as oils at all - the walls and benches are often left untreated.

There are products especially designed for that however, mostly based on paraffin, such as paraffin oil. I would look at similar Finnish products, the finns know their saunas.
posted by rpn at 9:32 AM on June 28 [3 favorites]


Doomed quest, I fear. Everything smells like something, especially when heated to sauna temperatures.

The best you can hope for is something that smells inoffensive and not headache-inducing; undiluted raw (not boiled) linseed oil (aka flaxseed oil) is what I'd use.

Fire up the sauna to get all the woodwork nice and hot, then brush on raw linseed oil, then rub it over with a cloth, then leave it sit for a day. Then do all that again, two more times. Then fire up the sauna and let it cook for a couple hours, then prop the door open with a pedestal fan blowing in to ventilate it aggressively as it cools down again. Keep doing that for a week. After all of that, the oil should have cured to the point where it no longer feels even slightly sticky to the touch and it's then ready for use.

The advantage of using raw linseed oil is that there's nothing toxic in it. "Boiled" linseed, and various commercial finishing oils based on it, include metal salts to help it cure faster that, given the choice, I wouldn't choose to heat up and then press onto my skin. And as a drying oil, it cures to a tough and durable surface-protective finish that mineral oils don't.

Commercial oil-based wood finishes all include various volatile organic compounds used to thin the oil and improve penetration into the wood, but these are not needed for your sauna application because making the wood hot will do both of those things every bit as well; and avoiding thinning agents means not having to deal with the smell of them evaporating.

As a bonus, raw linseed oil is way cheaper than anything commercial. The main reason it doesn't get used more is because it does take a fair while to cure properly. Again, heat is your friend here.

Just be careful with the cloths you use to rub the benches over; you don't want to wad those up because the curing reaction for a drying oil can release enough heat to make them spontaneously combust if it isn't allowed to dissipate. Spreading them back out into a single layer and pegging them up on an outside line is the best way.
posted by flabdablet at 9:38 AM on June 28 [2 favorites]


we typically don't use any finishing such as oils at all - the walls and benches are often left untreated

Every sauna I've ever been inside that features untreated wood has been made from cedar, which is an oily wood to begin with. Ash isn't.
posted by flabdablet at 9:44 AM on June 28 [2 favorites]


Please don't apply any finish...or at least use the sauna for a while and enjoy the lovely, clean cedar and ash smell before deciding. The benches will stay clean if you sit on towels. If you ever really want to clean them down the road, they're usually built to be removable and you can rinse them with soapy water. Once my ex’s idiot new-age friends decided to use some sort of essential oil on the rocks and ours was never the same again.

You lucky, lucky person.
posted by bonobothegreat at 10:01 AM on June 28 [1 favorite]


enjoy the lovely, clean cedar and ash smell before deciding

If your sauna does have a cedar interior, the terpenes released by that when it's heated are going to dominate the smell of the space almost exclusively. Without competition from thinners, or insanities like essential oils whose main purpose is to stink, you'd be hard pressed to smell anything else over the smell of hot cedar. Cured raw linseed has a much subtler scent than that.
posted by flabdablet at 10:10 AM on June 28 [2 favorites]


The hot cedar sauna smell is, to me something that's best enjoyed without adulteration.

I've used raw linseed and polymerized linseed oil on my woodworking projects and to me, they smell. I'd also say that an oil on the inside of a sauna is like oil on the inside of a dresser or cabinet - it's not appropriate to the utility and it'll be sitting enclosed all the time and develop a weird musty smell.
posted by bonobothegreat at 10:36 AM on June 28 [1 favorite]


There is no cedar, oak and ash only inside. I think I will not finish, I just didn’t want big ass-prints on the bench but pride goeth before an odor I guess.
posted by InkaLomax at 11:05 AM on June 28 [2 favorites]


Can you lay down some thin cedar planks on the seats?
posted by ananci at 11:18 AM on June 28 [1 favorite]


Good call to go unfinished. Personally I would be pretty wary of going into an enclosed space at 90C and breathing in who-knows-what outgassing from the finish.
posted by slkinsey at 11:39 AM on June 28


You're good with towels for the seats but might want to invest in some simple (removable) back rests to be more comfortable and keep oily back stains off the walls. Glad to read that you're not going to do a finish...now I can enjoy my Sunday.
posted by bonobothegreat at 11:48 AM on June 28 [2 favorites]


If you are willing to reapply the finish with some frequency, a soap finish would look nice on ash and impart some stain resistance, but did I mention the reapply frequently part?
posted by Glomar response at 2:28 PM on June 28 [1 favorite]


Just be careful with the cloths you use to rub the benches over; you don't want to wad those up because the curing reaction for a drying oil can release enough heat to make them spontaneously combust if it isn't allowed to dissipate. Spreading them back out into a single layer and pegging them up on an outside line is the best way.

Indeed. I nearly set my parents' garage on fire this way one summer when I was home from college.
posted by Johnny Assay at 5:46 PM on June 28


In case you decide to finish after all, please be aware that raw linseed oil can produce a very unpleasant fishy odor as it cures, which can take a long time to go away (I'd imagine this would be worse in a small enclosed space). If it were me, I'd stick with unfinished for a sauna.
posted by ourobouros at 4:14 AM on June 29


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