Use one coat of an oil- or alkyd resin-based primer for redwood. Backprime. Prime all ends, edges and sides. As an alternative to oil- or alkyd resin-based primers, use two prime coats of a waterborne "stain-blocking" primer specifically formulated for extractive rich woods. Work primers into the wood with a brush or roller, and allow to dry (according to manufacturer's recommendation) prior to finish coat application.
Do not allow wood to weather in an unprimed state. Weathering at this time reduces the paint-holding quality of the wood surface. If some weathering does occur, clean the wood surface and lightly sand to prepare it for priming and painting.
Protruding fibers on saw-textured surfaces act as channels for moisture, resulting in extractive staining. Brushing a minimum of two coats of primer into saw-textured surfaces assures a continuous, moisture-resistant film. Spray applications do not apply a continuous film on saw-textured surfaces, so they should be followed by thorough back-brushing or back-rolling.
Paint manufacturers recommend application rates for the primer, in terms of square feet of surface covered per gallon. These instructions should be carefully followed. Some painters thin the primer too liberally on the job to extend its coverage per gallon. Avoid this as it reduces the capability of the primer to do its job properly.
If the siding has been prefinished with a paintable water repellent preservative solution, prime it as you would bare wood before applying the top coats.
Some redwood suppliers offer pre-primed sidings, trim and fascia. Pre-primed redwood has considerable advantages over job-site priming. High quality factory-applied prime coats can seldom be duplicated in the field. Factory application of primers control two of the most important variables which determine paint performance: the characteristics of the wood substrate and the quality and application of the primer. The wood for factory priming is carefully selected, handled and stored prior to priming to ensure a clean, dry surface on which to apply the primer. By controlling application rates carefully, the factory can apply an even primer coat of known thickness. The benefits of factory pre-priming make it the preferred choice whenever possible.
When pre-primed material is cut during field application, the cut ends should be field primed prior to installation. This frequently overlooked step is very important since moisture moves much more rapidly through the end-grain than through the face of wood.
Factory pre-primed surfaces should be painted within 30 days of installation, or dirt, moisture and chalking may prevent bonding and shorten the life of the paint film. If the primed surface is allowed to weather for an extended period of time, clean and reprime the surface prior to topcoat application.