Horseshoes are usually tossed out after they've been used, making them cheap finds for individuals wanting to use them for games or art projects. There's usually an obstacle, though -- the buildup of dirt and rust can make them unusable for certain situations. Cleaning horseshoes is easier than you might think, and if you have a power grinder you'll be able to do it quickly, too.
Remove as much loose dirt and debris as possible with a toothbrush. Due to the sheer weight of a horse, compacted material likely is in all of the crevasses. Go over several times until all of the debris is completely removed, as this will greatly speed the process later on.
Remove any rust on the horseshoe. If there is only light rust, you may be able to get away with a thorough application of a rust remover purchased from your local hardware store. Follow the manufacturer's instructions and see if that is sufficient before moving to the next level.
Put on your safety mask and goggles if the rust remover was unable to penetrate the rust. With your angle grinder or Dremel power tool, use a bit or sandpaper specifically designed for grinding metal. Be meticulous during this process, making sure every edge and every indention is ground down until the metal is shiny again.
Soak the rust-free horseshoe in a solution of one part bleach and four parts water for two or three minutes. Dry completely and treat with a rust prevention product. Clipper or knife oil will work just fine as well.