I'd imagine with prolonged use they start to look crappy from the junk they get on the car. Do you just skim off a bit when it gets dirty and continue? Any tricks to storing it?

 

I use whatever piece I tear off one time only. Not worth rubbing some piece of crud embedded in a used piece of clay.

 

How much clay do you need to do a car? Just a small bit? They seem pretty small as is...

 

how much depends on how much crap is on the car.

e.g. the zaio package is two bars -
i can do my Bonneville with one bar - after an entire Canadian winter -

i also just tear off little pieces for spot work -

it can be stored by wetting it with a little Z7 soap - and then wrapping into plastic wrap - and putting back into the plastic container it came in.