The easiest way to clean the metal surface before finishing it is taking the trowel to the sandblasting cabinet. The way I like to think about it, is that it is a pressurized rapid sanding tool. Using compressed air and aggregate material to blast of the top surface of metal or whatever object you are sanding. Highly efficient at removing rust or grime, this tool is crucial for convenient finishing such as a patina. powder coating or painting.
Since the trowel is so small. it does not take very long to clean off the entire surface. Be sure to be careful of blasting the wooden handle! While there is a later step to sand off the original finish, the sandblaster will remove a bit too much material from the tool. At TechShop, it is standard procedure to wear some sort of gloves (I just used nitrile myself) when using the sandblaster. Since it is one of the tools in the shop that get used often, the dirt and grime from projects builds up in the cabinet's gloves which is unsanitary. The tool is pedal operated and it's basic use is covered in the SBU
Once the sandblasting is done, the metal will be cleaned off and will be a dull grey color, as opposed to the shiny quality it has after grinding. Keep in mind, if you are intending on putting an edge on the trowel as mentioned in the last instructable, be sure to sandblast first or be prepared to put another edge on it. (I learned this the hard way).
You are now ready to apply the patina!