How to Waterproof an Outdoor Storage Box

Co-authored by Christopher Lanier and Kyle Hall

Last Updated: August 30, 2023 References

You have a wooden box that would be perfect for outdoor storage, but there’s one problem: it’s not waterproof. Fortunately, by using varnish, you can turn any wooden box into a waterproof outdoor storage container that will stand up to the elements and protect your belongings. Keep reading for step-by-step instructions on how to DIY waterproof your outdoor storage box and achieve a beautiful finish.

1

Use marine varnish.

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Since your storage box is going to be outdoors, you need to coat it with something durable enough to withstand the elements. That’s where marine varnish comes in. This exterior varnish is specifically designed for wood that’s exposed to water (like the wood on boats). Marine varnish is also resistant to sun damage, meaning it won’t break down in the sun over time like other sealants.^{[1]
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2

Sand the storage box before waterproofing it.

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Sanding first will give the box a smoother, more even finish. If you’re not concerned about the appearance of your storage box, you can skip this step. However, if you want a smooth finish, go over the entire outer surface of the box with 180-grit sandpaper by hand until the wood is smooth and scratch-free.^{[2]
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- When you’re done sanding, remove leftover dust on the box—you don’t want that getting stuck underneath the varnish and ruining the finish.
- Vacuum up as much dust as you can using a brush attachment, then wipe down the box with a cloth lightly dampened with paint thinner.
- Finish by wiping down the box with a painter’s tack cloth.

3

Thin the varnish with paint thinner.

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Using thinned varnish for the first coat will help it penetrate the wood. Open your can of varnish, gently stir it, and pour some into a plastic container. Then, add enough paint thinner so you have about 10 parts varnish to 1 part paint thinner. Use this thinned-out varnish for the first coat on your storage box. That way, the varnish will really seep into the pores of the wood and create a durable, protective base.^{[3]
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- Your specific brand of marine varnish might recommend a different varnish to paint thinner ratio. Follow the instructions on the can for the best results.^{[4]
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- Be gentle and go slowly when stirring varnish. Stirring too quickly will create air bubbles, which will affect the finish.^{[5]
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4

Apply the first coat.

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Use a paintbrush to apply the thinned varnish to the outside of the box. Dip your brush in the thinned varnish, then use back and forth strokes to coat the wood. Keep going until the entire exterior of the box is coated with a thin, even coat of varnish.^{[6]
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- Work in a clean, dust-free space. Dust, hair, and other debris stick to varnish and ruin the finish.^{[7]
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- When you're working with marine varnish, always dispose of rags, sanding residue, and other waste in a sealed metal container filled with water. Items that have come into contact with varnish can spontaneously catch fire, even without an ignition source.^{[8]
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5

Let the first coat dry.

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Varnish needs to fully dry between coats for the best results. The exact dry time between coats will depend on the brand of varnish you’re using. Some brands suggest waiting 8-16 hours between coats, but check the instructions on the can for a specific drying time.^{[9]
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- Temperature, humidity, and the thickness of the varnish can all affect the actual drying time.

6

Lightly sand the varnish.

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Sanding between coats will give you a better finish. It seems counterintuitive to sand a coat of varnish you just spent time applying, but doing so will remove imperfections and help the next coat adhere better. Once the first coat is dry, go over it with 220-grit sandpaper by hand to lightly scuff it and dull the shine. Use a light touch so you’re barely removing any varnish.^{[10]
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- If your sandpaper gums up while you’re sanding, the varnish isn’t dry enough.^{[11]
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- When you’re done, clear off dust the same way you did when you sanded the bare wood. Use a vacuum with a brush attachment, wipe down the box with a cloth dampened with paint thinner, then go over the wood with a tack cloth.

7

Apply at least three coats total.

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For maximum water protection, you’ll want to do multiple coats of varnish. The exact number of coats you should do will depend on the brand of varnish you’re using (it should specify the minimum number of coats in the instructions) and your desired finish.^{[12]
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Research source} Generally, you should aim for at least 3 coats. For subsequent coats, use full-strength varnish (not varnish mixed with paint thinner).^{[13]
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- Some varnishes recommend thinning the second coat with paint thinner as well. Follow the instructions for your specific brand of varnish to get the best finish.

8

Let the varnish cure.

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Wait at least 24 hours to use your storage box. Then, be gentle with it during the first week after varnishing so it has time to fully cure and harden.^{[14]
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Research source} Once the varnish has cured, your storage box is officially waterproof and ready for the elements.

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Tips

Avoid varnishing when it’s humid out. Moisture can affect the finish.^{[15]
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Wait until the morning dew has evaporated to apply a coat of varnish. Similarly, don’t varnish two hours or less before sunset.^{[16]
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Apply the varnish on a cool, dry day if possible.

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Warnings

Rags, sanding residue, and other waste that has come into contact with marine varnish can spontaneously catch fire, even without an ignition source. To avoid this, dispose of these things in a closed metal container filled with water. Check your local fire regulations for more information on the best way to dispose of waste from this project.^{[17]
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Inhaling dust while sanding wood is bad for your lungs. Always wear a mask while sanding.^{[18]
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