Cleaning a worn bath

We have an old bath here where the enamel has started to wear. I really struggle to keep it looking clean as the lower half tends to look dirty.

I found bleach useful, I diluted it in a spray bottle first. I found it took multiple sprayings, plus the brand I was using was not septic safe. Switched to a septic safe one and it turned the whole bottom of the bath rust orange :o

We will be getting rid of this house in a few years, so we wont be replacing the bath tub or paying for it to be re-enamelled (well not unless anyone knows a cheap DIY method). So how can I keep it looking clean without hurting our septic system?

try bicarb and vinegar.

Does absolutely nothing on our bath.

Sugar soap???

And lots of elbow grease.... :(

Is shower power septic safe? I used to use that on our bath.

Oh, also, I reckon Jiff or Ajax Cream cleanser is really good. Not sure if it is septic safe but I cant see why not.