Mix trisodium phosphate (TSP) and water according to the package directions. TSP is a powerful cleaning solution and also will dull the shine on the surface of the cabinet. Dip a clean rag into the mixture and wipe down the filing cabinet. Remove the drawers from the filing cabinet and set them aside.
Turn the filing cabinet over and set it on its top. Set casters 1 inch from the edge of each corner. Hold the caster in place with one hand and mark the holes in the installation brackets with a permanent marker.
Drill holes through the marks. Drill through the metal slowly and pull back if you feel the bit getting stuck. Drill all the necessary holes to attach the casters. Set the casters on the bottom of the cabinet and screw them into place. Choose 1-inch screws or any screws that are short enough that they won't interfere with the bottom drawer of the cabinet. Slide a bolt onto the end of each screw to hold it in place. Turn the cabinet over onto the casters.
Trim a piece of butcher block counter so it is 2 inches wider and longer than the top of the filing cabinet. Trim the butcher block with a circular saw. Set the board on top of the cabinet. Hold the drill inside the cabinet and drive 1-inch screws through the metal into butcher block.
Slide the drawers back into the filing cabinet.