At the chemist I buy a piece of volcanic ash type stuff. It is in a block like a cake of soap and then packaged in a box and it is only a few dollars. You wet it and out soap on it and scrub away and it takes out my cracks and the dirt (sounds weird put like that....). The block wears away as you use ut and leaves little bits of ash on the shower floor that go down the drain...
May be worth a try?