Choose the right tool for the job. Tell the paint dealer what types of surfaces you'll be painting and ask him to help select the proper tools for the job. Use the widest brush you can comfortably handle on wide siding or large areas. Use a tool you can handle for hours, without fatigue. Don't overthin paint.
Paint goes on easiest and lasts longest when it is applied as formulated.
Paint that is too thin will run, sag, drip and spatter, and protective qualities may be sacrificed. Most manufacturers recommend that to ensure durability you limit thinning to not more than one pint of water per gallon of paint. Most paints recommend a spread rate of 350-400 sq. But be aware that the coverage can depend on the surface to be painted. Trying to extend the coverage by thinning or excessive rolling of the paint will result in a too-thin paint film that will fail prematurely.