Good preparation is the key to a successful paint job, especially with exterior surfaces.
 
The goal is to create a smooth, clean surface to apply paint on. Ideally, prep and prime your surfaces in sections, and at a time when you can immediately paint that section. (Exposing a primed house to moisture for long periods of time before painting will ruin all your efforts.) Follow these steps.

Clear debris. Remove all extraneous items, such as outdoor furniture, planters, mailboxes, shutters, window screens, storm windows and light fixtures. Tie or cut back trees and shrubs to make room for ladders.

Smooth the rough spots. Using a scraper, remove peeling, cracking or deteriorating paint thoroughly. Sand glossy surfaces with a 60-grit or 8-grit paper to roughen the surface so paint can adhere. On iron and steel, rust should be sanded or wire-brushed away. Sand rusty nail heads, then sink them down slightly beneath the surface with a hammer and nail punch. Apply corrosion-resistant primer over the nail head and add putty if necessary. Then sand smooth. Wipe the entire surface with a tack cloth to remove sand particles.

Caulk the cracks. Check out your siding and open joints between materials. But prime unpainted surfaces before caulking.

Remove the resin. If you find resinous material on wood knots, remove the resin with turpentine or mineral spirits, then seal with a special knot sealer available at your paint store.

Use filler. Unpainted masonry surfaces that are particularly rough should be filled with masonry block filler—a thick material that creates a smoother, less porous surface.

Clean up surfaces. Wipe off dust or dirt with a cloth before thoroughly washing surfaces. Use a detergent recommended for washing paint and removing oil and grease. Include areas you sanded to remove soluble salts. A long-handled brush will help you get to hard-to-reach places.
Clean up mildew. Look for mildew stains, especially under eaves and other damp areas that do not receive sun exposure. Use a mild bleach detergent to remove stains and rinse well with a garden hose, using a strong stream of water. After the surface of the eaves has dried, apply self-priming IronClad® Latex Low Lustre Metal & Wood Enamel

Prime exposed areas. Check all exposed areas for thinning, peeling or blistering paint. Apply primer and allow it to dry thoroughly.

Gutters and Downspouts. Clean and prime your gutters and downspouts before painting. Remove any rust or peeling paint with a scraper or wire brush. Sand the edges smooth, from sound paint to bare spots.