For most acryilics, I thin them with rubbing alcohol. Alcohol evaporates fster than water. Like as has already been said, priming is just a neutral undercoat of a single color that gives the plastic surface some "tooth", giving the color coat something to stick to... I just use plain old enamel grey primer from Wal-Mart. The cans are only a buck or so. Since you're using acryillics, the color coat won't dissolve the primer. Also, if you have to put more than one color-coat on, you should seal the first coat with clear flat to keep the second coat from dissolving the first one.