Another one. If you are painting something like an engine block, or some other piece that you don't want to get paint on machined surfaces, take a thin film of grease and apply it to those area that you don't want the paint to adhere to. Paint your project, let it all dry very well, and then take a rag with acetone, brake cleaner, or your poison of choice and clean the grease and overspray off. Sometimes this is faster, easier, or works better than traditional blue masking tape.