Wrap the sandpaper around the block of wood and lightly sand all edges of the steel plate checking for any sharp burrs. You only need to take a few swipes with the sandpaper to knock down any sharp edges.
In a container large enough to hold the steel plate, lay the 4 pencils or any small object to act as a spacer and prevent the steel from touching the bottom of the container. Place the steel onto the spacers.
Pour 1 gallon of vinegar and 1 gallon of water into the container making sure the steel plate is completely submerged. If the steel is not submerged add more vinegar and water in a 50/50 ratio to completely cover the steel.
1 gallon distilled vinegar, 1 gallon water
After 48 hours wipe off all the mill scale with paper towels and if needed use the scoring action of an abrasive sponge to remove any tough bits. When finished wiping the scale off, rinse with water then dry off immediately to prevent any surface rust from forming.
Quickly place the dried-off steel in a 170f oven to remove any remaining moisture.