Clean the surface of your outdoor decorations to remove any mildew, debris, rust or other stains. A power washer will remove mildew, loose particles and some stains, but you'll need to scrub rust with a wire brush to remove it. Depending on the type of material you are cleaning, a 50/50 mixture of bleach and water may help remove stains. Sand wood surfaces if they are rough with a hand sander and medium-grade sandpaper.
Repair any imperfections that may allow the sealant or waterproofing paint to sink into the surface of your decorations. This would include a hole in a concrete statue or an open notch in an outdoor wooden planter. Wood fillers and concrete repair kits are commercially available for small repairs. Follow the manufacturer's directions and allow the repair medium to dry or cure completely before you apply the waterproof material.
Apply the appropriate waterproofing medium for the type of construction material that your outdoor decorations are made from. You may prefer to use a block and wall sealant for outdoor concrete structures and a clear waterproofing sealer for wood or plastic decorations. Waterproofing paint works on a variety of surfaces. Follow the manufacturer's directions for each application.
Paint over clear waterproof sealer with latex or oil-based paints to add another layer of protection as well as a design element to your outdoor decorations.