You should thoroughly clean any surface before applying paint products, this means soap, water, scrubbing, whatever it takes! Exterior surfaces should be cleaned of dirt, sap, mildew, and any other substances that might inhibit adhesion or take away from the overall appearance and durability of the finish. While exterior surfaces can be cleaned by hand with household or commercial cleaners, this tends to be time consuming. A more conventional way to clean exterior surfaces would be to use a pressure washer , as it is not time consuming and will achieve better results than hand washing with soap and other cleaners probably will.

Obviously different surfaces require a different type of pressure washing and the amount of pressure that is used may very from surface to surface. If used improperly, pressure washers can cause a lot of damage to surfaces. So, know what you are doing so you do not ruin your home exterior.

When it comes to interior painting, cleaning the surfaces is just as important. Although they will probably be easier to clean, as they are not subject to the elements like the exterior is, it is not any less important to clean them properly. All interior surfaces should be free of dust, dirt and grime. Scrubbing may or may not be necessary that depends on how dirty the surfaces are. Generally you do not need and special cleaners, just the use of general household cleaners should be adequate. However, you could use a special wall cleaner if you wanted to.

The next thing you need to do for surface preparation is repair all gouges, cracks, stains, holes, etc., and then sand the areas smooth. This has to be done before applying primers and paints.

If you are painting over existing paint, then after the surface is thoroughly washed, you will want to be sure to remove all loose and peeling paint, if any. You do this by sanding the areas where paint is peeling or cracked. This is done to provide a smooth surface. The degree of sanding will be determined by the thickness of the paint. All damaged area's need to be repaired, as any imperfections will show through the paint. Paint is not a big cover up like foundation is in make-up. You do not put it on the walls to cover problems, problems like cracking and holes will still show, so to get a good looking paint job you need clean, dry, smooth surfaces to paint.
So, with the goal of clean, smooth, and dry in mind be sure to remove all rust stains, and caulk any cracks. Rust should be wire brushed and treated with a hardening chemical and a top quality rust inhibited primer to prevent future rusting. Rusty nail heads need to be reset, primed with a top quality rust inhibited primer and caulked. If rust is not properly treated it will reemerge through the new paint within days. If you have excessive cracks, be sure to caulk them, and then apply a top quality primer and a top quality finish coat.

Once all of your surfaces are clean, dry, and smooth, it is wise to apply a coat of a top quality alkyd in order to provide maximum adhesion.