When you use steel clamps to hold wood components together and the clamp face is wetted by the glue, it sometimes leaves a black discoloration on the wood surface (for some glues). That is very difficult to remove, heavy sanding is the only solution I have found. This apparently is the result of a chemical reaction between the steel and the glue. This problem doesn’t occur when using aluminum clamps. Placing a piece of aluminum foil under the clamp face where it can touch the wet glue prevents this problem.
– John Sturtz