Easiest way to clean your bath tub. Buy the cheapest oven cleaner you can (Dollar Store) and spray the tub. Let sit for 5 to 10 minutes and all the scum and gunge comes off. If it's really dirty may have to repeat.
This will NOT harm your tub no matter what it's made of. I've been doing this for years and swear by it. I also do my bathroom sinks too......That is when I clean....