There are a variety of different powders out there to put in an air abrasive unit. The most common in fossil preparation are dolomite, aluminium oxide, iron powder and sodium bicarbonate. These vary in effect as a result of the differences in grain size, grain shape and hardness. Many professional preparators will say that it is worth getting to know a powder and knowing it really well. You can then achieve just about anything with said powder by understanding which pressure and angle to use it at. All powders are capable of obliterating detail on a fossil, but it is worth trying to match the powder to what you are preparing. Some powders are more forgiving than others on different surfaces, and so experimentation and experience are the key to success.