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I built a large planter using untreated wood. Before pouring the potting soil into the planter, I have to paint the the planter with something that will protect it from the soil, the humidity, etc.

I would need to paint the interior to protect the wood from the soil, but also doesn't poison the soil and/or the plant.

I would also need to paint the planter exterior to protect it from the elements (sun, rain, termites, etc).

What do you suggest I paint it with? I prefer painting the whole thing with just one paint, instead of using two paints.

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    Nothing you apply will prevent rot inside the wood. You can't get that kind of protection from a brush-on surface treatment. Moisture will work its way in and do its thing. This is why pressure-treated lumber exists, wherein the anti-microbial treatment is pushed to the core of the lumber. – isherwood Aug 27 at 18:31
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    If you want to protect your wood planter from moisture entering from the soil inside it, then you'd need to line it with am impervious layer/sheet or some sort, like perhaps a matching plastic bin/box which fits inside or gluing plastic sheeting (folded at the corners, not cut). – brhans Aug 27 at 18:42
  • you need a liner inside, no way around it. – dandavis Aug 27 at 20:27
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I'm not saying this is the best, perfect solution, but if you do want a paint to protect the inside of the planters and not harm the plants, you can use a spray on "pruning seal" product. It's designed to seal up large wounds in a tree, but the spray can will also mention spraying the inside of a planter to waterproof it. I've used it before to waterproof the inside of a wooden bowl to create a small cactus garden.

You can apply several light coats to build up thickness. If you try to spray it too thick, it takes forever for it to dry.

(randomly picked example image, comes from several brand names and is the same stuff as far as I can tell)

Prune seal

  • Trouble is, this will tear at any joints due to movement. After that, all is lost. – isherwood Aug 27 at 20:03
  • @isherwood, true but depends on how the planter is constructed. Small joints, and very stiff - it has a chance to lengthen the life better than other products. Continuous plastic sheet would probably be best in other cases. – JPhi1618 Aug 27 at 20:06
  • Sure, but you'd have to wrap the entire thing, inside and out. It's outdoors, after all. – isherwood Aug 27 at 20:08
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LeakSeal, pruning seal, roof cement, or an outdoor epoxy finish would provide the most lasting durability. Each is prone to eventual failure though.

If the entire thing is constructed of 2" thick wood, it would probably last a few years before rotting through left as-is. I know organic gardeners sometimes use normal 2*6s to build raised beds.

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