How to get better looking results with Sandstone texture paint

Here is a short HOWTO article that answers some of your questions: http://tlc.howstuffworks.com/home/painting-interiors9.htmt

Here are a series of HOWTO's from BEHR's product pages: BEHR's Texture Painting HOWTO's

Here is another one from a specific product page: http://www.skalflex.com/products/texturePaints/interior.html

Sand evenly across the entire board to make it smooth. Clean and remove all dust from sanding. Respray as evenly as possible. If you can get away with doing it indoors (a garage for example) that is recommended to reduce the number of airborne particles that land in the paint before it can dry.

To ensure an even coat my personal method is to gauge the coverage by sheen rather than color. Meaning that if you view it at an angle the wet portions will be shinier than the less covered portions. Color can be deceiving, look at the sheen, if the sheen matches (as you spray) across the piece, the final product will be more even as well.

Keep in mind that with spray paint it takes less time to do extra coats of paint than it does to put on too much and fix a poor result...tempting though it may be to try.