 It was typically applied as a casting "in situ" in a layer about thick on to existing ship structures made from mild steel or formed in equally thick sections on a steel plate for mounting as gun shields and the like.Plastic armour replaced the use of concrete slabs which, although expected to provide protection, were prone to cracking and breaking up when struck by armour-piercing bullets.Plastic armour was effective because the very hard particles would deflect bullets which would then lodge between the plastic armour and the steel backing plate.