Outdoor planters are subject to all the whims of the weather, no matter where you live. You can give them some protection against the weather by sealing them. You should seal your planter before you install any soil or plants inside them.
Step 1 - Confirm Sealing is Required
You don't need to seal most ceramic planters. If the planter has a glazed surface, you don't need to seal it unless the finish is damaged or cracked. Garden planters made of cement or concrete do not need sealing.
You may treat porous surfaces with a suitable sealant. Apply the sealant lightly with at least two coats. Allow the sealant to dry between coats. Ensure the planter has ample drainage on the bottom.
Step 2 - Disassemble and Seal Wood Planters
The best way to ensure a complete seal of a wooden planter is to take it apart and seal the individual pieces. Apply the sealant in at least two light coats. Use a soft paintbrush. Allow the sealant to dry between coats. Pay particular attention to the ends of each piece because that is where they are most vulnerable.
Step 3 - Apply an Extra Coat to the Base
Apply an extra coat of sealant to the base. If it contains no drainage holes, create them before you apply the sealant.
Step 4 - Reassemble Outdoor Planters
When you have sealed all the parts and allowed them to dry, rebuild the outdoor planters.
Step 5 - Varnish the Outsides
To complete the seal, apply a varnish to the outside surfaces of the planter. This will give additional protection against water and dirt. A clear varnish will enhance the natural color of the wood, while a colored varnish can give it a different appearance altogether.
Step 6 - Discard Metal Trays
Some wood planters contain metal trays that fit into the bottoms to act as drip trays for potted plants. If you seal the planter properly, you can and should discard this tray.
Step 7 - Paint Concrete Planters (Optional)
If you want to paint a concrete planter, you will need to seal it with at least two coats of sealant; otherwise, the paint will peel off. If you seal concrete planters, you should install drainage to protect the plants. This process can be as simple as putting a few inches of gravel in the bottom of your outdoor planters before you fill them with soil.
Sealing outdoor planters can extend their useful life and help to keep them in good condition. Before taking the time to apply a sealant, make sure you will not be introducing a problem. You don't want to interfere with the natural drainage provided by the material of the outdoor planters.