Walls should be wet down before getting scrubbed, then washed with a gallon of water mixed with 1 cup chlorine bleach and 1 cup of either a concentrated, phosphate-free cleaner, such as a trisodium phosphate (TSP) substitute, or Jomax House Cleaner. Working in sections, from the bottom to the top, will avoid streaks. Be sure to rinse walls well before the solution dries. Wood siding and trim should be ready to paint after a day or two of dry weather.

Once you have the interior square footage calculated, experts say you’ll want to divide by 350. 350 square feet is the average coverage of a gallon of paint (though some say 250-400 square feet). The average coverage for a gallon of primer is 200 square feet. Your square footage divided by 350 square feet gets you the number of gallons of paint you’ll need. Square footage divided by 200 gets you the number of gallons of primer you’ll need. You can save money by ensuring you don’t buy too much paint. House Painting Thornton

While you may choose to paint a door, ground-level window trim and other parts of your house’s exterior, it’s really a job best left to the professionals. One of the main reasons is experience. An experienced painter knows how to handle the prep work, spot problems and identify sources of future trouble, all while getting the job done more efficiently. They are also experienced with spending long hours on tall ladders. If you want high-quality work, it’s best to hire a professional painter near you. 
At least 90% of painting time goes into setup, prep work, and priming. The method of prepping required varies for each type of exterior material on the home, but every project must be patched and cleaned. The more thoroughly you prep and prime your home’s exterior, the longer your paint job will last. You could save quite a bit if you do this part yourself, but it must be done correctly, or your contractor will have to go over what you missed. The prices mentioned are based on paying a contractor to do each job rather than one overall quote for the work from start to finish.
In addition, paint features such as mold and mildew resistance are important cost factors. Many mold-inhibiting bathroom paints, for instance, cost substantially more than ordinary latex-acrylic paint—sometimes close to twice the cost. Also, paints that come with warranties from the manufacturer may be more expensive depending on the length of the warranty. (A two-year warranty is standard.) Thornton House Painting
In the same way that you need to be aware of pet activity, your exterior paint prep project will require you to keep shrubs and plants in mind as well. Cleaning paint debris out of bushes is no fun, so use a lightweight canvas tarp to cover up flower beds and shrubbery. But plan on removing the protective covering at the end of each day so that you won't risk killing off the plants you're trying to protect. As you place your ladders and drop cloths, keep an eye open for delicate plants or flowers. Avoid them if possible or if they're fragile, consider relocating them into temporary pots. And definitely protect any water features such as koi ponds or waterfalls. A little bit of preplanning can save lots of aggravation and repairs down the road. House Painting Thornton
Once you have the approximate square footage that you’ll need to paint, remember that a gallon of paint generally covers between 250 and 400 square feet. You’ll need more if you’re painting a rough, porous material, or if you want to paint a dark house a much lighter color. Depending on your color choice, you may need a more expensive tinted primer to help with the color transition. Thornton House Painters