It's a very nice lesson you're teaching here!!! But on da oda hand I've been making series of dis and i find it difficult to remove da inner plastic after removing da first one... Da hole tin end up breaking.... I use Engine oil, sharp sand, cement and water
I've noticed that your plastic rubber always have dis handle to enable u remove it... But mine get swallowed by the cement so I'll have to cut a little bit of bfor removing and it will still scatter