The metal ready goes in, cover the hole and slosh it about.
Then just sit the frame down and let it dry, if some leaks out it won't matter, equally if it all stays in as a pool of fluid it will eventually dry. Metal ready is a rust converter, it stops the chemical reaction and leaves a film to protect.
The electrox removes rust and leaves a bare metal surface needing coating, It can't be left in or it leaves the rust in with it and then it's a real pain to remove.