How to Waterproof a Wooden Box

Wooden boxes have a wide range of uses; they can be used for storage purposes especially if you have old belongings that you don’t want to throw away. They can also be used to plant a wide range of flowers and other exotic plants.

The reason wooden boxes are used as a planter’s box is because they blend well with nature due to its natural appearance. Wood is permeable, which means it soaks up water. When this happens, it rots and compromises the structure of wood, which weakens it. Water is not the only element that contributes to wood damage. Ultraviolet rays from the sun are known to be harmful not only to  human skin, but also to wood.

To protect the structure and integrity of a wooden box, you need to waterproof it. The process is not difficult, and you can accomplish the steps using a few supplies which can be found in your garage. Other items can be bought at a hardware or home improvement store.

What you need

  • 220 grit sand paper
  • Silicone caulk
  • Oil-based varnish
  • Precision knife
  • Plastic sheeting
  • Paint brush
  • Power drill

How to waterproof a wooden box—step-by-step guide

1.  Prepare the wooden box

A majority of wooden boxes are built with pressure-treated or untreated lumber. To prepare the box for waterproofing, you need to set it on a flat surface in a room. Let it rest for two to three weeks, especially if it has recently been assembled. This is done to allow the wood to dry properly and not seal in moisture. The waiting period also helps prevent the need for sealing joints in case they were to expand.

Treated wood is known to last longer, but it contains chemical preservatives, so you will need to take more care when cutting. If you are going to use the wood as a planter box, drill 4 to 6 holes at the bottom of the box, especially the central plank. They should be ¼” in diameter. This allows any excess water to drain from the soil of your plant.

2.  Apply silicon caulk

Before starting, you need to protect your hands. Wear tight-fitting disposable gloves before coming into contact with the silicon caulk. Why? This helps to prevent the caulk from coming into contact with your skin, which could be a problem if your skin is sensitive.

Take the silicon caulk and apply a bead to fill any cracks in the wood and joints. Using your gloved finger, drag the caulk along the cracks and joints. You can also dip your gloved finger in water to help in removing the excess caulk and allow creation of clean lines. Let the caulk completely dry. This may take a few hours to a few days.

3.  Apply oil based varnish

Mix the oil-based varnish according to the instructions provided. Using your brush, apply the varnish to the inside and outside of the box; remember to have your gloves on when doing so. Apply the varnish all over the box and make sure you have added extra varnish around the drainage holes.

You need to do so until the wood stops absorbing the oil-based varnish. It is important to keep the wood wet with varnish for 10 minutes. Using a cloth, wipe off any excess varnish. Once you are done, allow the oil-based varnish to cure for 24 hours.

4.  Sand the box

After the single-cure day has elapsed, sand the box. Proceed to apply a few more coats— probably two or three. Make sure that you have allowed 24 hours of curing between each coat. If you are using the wooden box for planting, then you have to line the interior with the plastic sheet.

Cut the plastic sheet with the precision knife if necessary. Fold the plastic sheet to double the thickness. From here, you can proceed with seeding and planting. Make sure the drainage holes at the bottom are not covered by the plastic sheet. You can also use a mesh screen over the drainage holes.

5.  Final steps

If the wooden box is used for storage, ensure it is above ground. You can use a metal rack to suspend the wooden box.

That is how you waterproof a wooden box.