I am preparing a mold to encase an 8×8×½-inch mild steel plate in epoxy. I have done this before. There is now curing silicone caulk, joining pieces of HDPE ("plastic cutting board material", if you want).
Such steel plates make nice heavy weights, but epoxy keeps them easier to handle and protects from corrosion.
You can use Evapo-Rust to remove milling scale and rust.
#alt4me