Concrete planters come in varied shapes and sizes. Precast concrete is a porous material susceptible to weathering much like terracotta or clay pots. The interior portion of a planter receives constant exposure to organic materials in soil and moisture from watering. Sealing concrete planters extends the life of the container to provide long-lasting beauty to a container garden.
Place a few drops of dish detergent inside the planter and spray a small amount of water onto a scrubber. Clean the interior and exterior of the planter thoroughly to remove dirt, grime, leftover garden soil and plant materials. Rinse the inside and outside and allow the concrete planter to dry for two to three days, in a sunny location with no further exposure to water.
Place the planter on a sturdy work surface lined with a plastic tarp or newspapers. Slip a wood block under each end of the planter to raise it up off the drop cloth surface. This eliminates sticking of the planter to the work surface when sealing the bottom edges of the planter.
Shake the container of sealant completely and pop open the top using a flat-edge screwdriver. Insert a long handled paint stirrer into the sealant and stir thoroughly down to the bottom of the can. Like all paints and sealants, concrete sealant particles settle along the bottom of the can and you need to mix the sealant before you use it.
Dip the paintbrush edge into sealant can and dab off any drips. Begin with the inside of the planter. Seal the corners and edges and work outward to cover the entire inside of the planter. Apply sealant in smooth, even strokes, much like painting a wall, to eliminate drips and blotches.
Move to the outside of the planter to apply sealant in a thin first coat. Pay particular attention to the grooves and ridges of ornamental etchings on the outside of the planter. Apply sealant down to the feet of the planter. Allow the first coat to dry based on recommendations on the can and apply a second coat to the interior and exterior of the planter.
Allow the second coat to dry for 24 to 48 hours before filling the planter with soil and flowers.