There are two reasons for sanding wood prior to staining. The most obvious reason is to make the wood smooth, but another important reason is to increase the porosity of the wood and make the penetration of the stain more uniform — especially with transparent stains.
Wood decks are commonly painted or stained after they have been constructed. After construction, the sides and bottom of the wood cannot be coated properly. Many failures on decks are caused by moisture entering the wood through the sides and bottom. Moisture seldom leaves these surfaces the way it entered. The sun shines on the coated surface and the moisture tries to come out through the paint or stain which causes peeling. To improve the resistance to moisture, the coating should be applied to all surfaces prior to construction.
Painting Over Varnish
Adhesion of most paints to varnish is poor. Although deglossing by sanding with #120 grit sandpaper can help adhesion, paints applied over varnish usually chip easily. For best results, strip off the varnish with paint remover and then prime with an alkyd primer before top-coating with alkyd or latex.
Pressure-treated lumber is kiln-dried wood (usually spruce or yellow pine) that is pressure treated with chemical preservatives dispersed in water. To check if the wood is dry enough for stain or paint, put water drops on the surface. If it soaks in, it’s ready. When the wood is dry, it can be painted or stained like any other wood surface. Pressure-treated wood that is not painted or stained will weather, crack, and check as badly as untreated wood over time.
Wood that has been exposed to the sun for more than a month should be sanded before painting — especially if the wood has started to turn grey. Sunlight causes the wood fibers to "chalk" and erode unevenly. They then form a weak surface layer that has poor adhesion to the wood surface, which can cause the paint to peel. To see how much sun damage has been caused to the wood surface, press a piece of tape on the surface, rub the back of the tape to ensure good contact, and then strip off the tape. If there are a lot of wood fibers on the back of the tape, the wood needs sanding before painting.
Grain cracking in plywood is caused by moisture cycling in and out of the top veneer. If plywood is going to be painted with latex paint or stain, two coats of exterior alkyd or high-quality acrylic primer are recommended first, in order to reduce the chance of cracking. Never use semitransparent stain on plywood. It does not provide enough protection from moisture and the plywood will crack and check.