Greased pied caps and feathered hats should lose charisma bonus

I realise that it's enchanced by magic, but a greased feathered hat makes you look like an idiot .... Maybe just reduced the bonus some, for well enchanted hats? Other idea -- only metal helmets protect against mindflayers, so they are the only ones worth enchanting. It does seem silly that you can greese a hood or a headband and get the protection. If greasing your hat damages it for charisma purposes, then 'repair armour' removes the grease?