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So I know the off-the-cuff answer is you cannot (or should not) use Indoor Rated Poly to weather proof wood as the sun's UV will discolor the wood and other issues visually. This wood though is being used to stack firewood and the way it looks doesn't matter at all. I have about 3/4 gallon of left over Poly and 3/4 Gallon wood stain. Both are indoor use. I'd rather just use those then go buy something new.

My question is can I just go ahead and stain/poly the wood and stack my firewood on top? Will it help? I'm using Douglass Fir wood.

I also have leftover exterior paint but I'd rather not paint the boards. Just a thought.

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  • If it will be in contact with the ground, it really should be pressure-treated lumber.
    – keshlam
    Commented Oct 6, 2016 at 21:35
  • depending on where you live, UT ply can last for years outside - it'll definitely fail way sooner that PT ply, but depending on the situation that may not matter to you. I've got some 18mm UT ply outside that had been given a couple of coats of varnish - 3 years later it looks manky but structurally it's fine. (and most of the varnish has peeled off)
    – Dave Smylie
    Commented Oct 6, 2016 at 21:51
  • Awesome, thanks guys. I may just give it a shot. I didn't want to use PT lumber because the wood is going on top of what could be a garden bed in a year or two... I move things around a lot. So I don't want those chemicals in my food. The planks were $10 total so if I NEED to replace in a few years not a huge deal. Thanks so much!
    – DigitalMC
    Commented Oct 6, 2016 at 21:57
  • why stain the wood if it's going to hold firewood? stain only colors, it offers no additional protection.
    – aaron
    Commented Oct 7, 2016 at 14:35

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Interior varnish used outdoors will certainly work to protect wood in the short term, but I don't think it's the right way to go here.

One reason is that a film finish like varnish will tend to transfer if pressed upon with significant force over an extended time. While that might be irrelevant to the wood being stacked it will peel varnish from the support, probably defeating the purpose of varnishing it in the first place.

If you're seeking a long-term solution to protect these fir supports from weather and decay, particularly if they're intended to be in direct contact with the ground, I'd suggest you use an old-timer's trick and soak them in used motor oil. This can apparently protect even softwoods from decay for many decades.

You'd soak the wood for a good while (a few days at least, but a week or more wouldn't hurt if possible) to let the end grain drink up as much oil as it can, then wipe off the excess and put them to use.

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  • Q: If soaking is impossible due to the size of the wood, would just painting (or possibly spraying) one or more coats of oil on be worthwhile?
    – martineau
    Commented Oct 13, 2016 at 15:54
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    @martineau It should be possible to rig up some temporary system to allow soaking of larger pieces of wood. For anything under the size of a black plastic bin liner/trash bag you can use one of those (or double up to ensure against leaks), dump in the wood and pour in the oil. For very large pieces you can dig a trench and line it with sheet plastic — this is in fact how the guy I learned this tip from did it. If soaking the whole piece just isn't possible sit the cut ends one at a time in a shallow container of oil to allow the end grain to soak it up & paint it onto the long grain surfaces.
    – Graphus
    Commented Oct 14, 2016 at 6:10

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