My bathtub has a textured non-slip surface on the bottom, which is in the form of a fine grid of raised and lowered areas, forming thousands of tiny squares about 1mm on a side. So each square is a little indentation in which dirt and grime tends to accumulate, and you can't wipe it away with a sponge.
So far the only way I have found to clean it is to spray it with a cleaning product (currently using Scrubbing Bubbles Shower Foamer) and then scrub it hard with a very stiff brush. It seems that the spray softens the dirt a little, and the brush bristles eventually dig into the indentations to scrape it out. But this is quite laborious - you have to scrub quite vigorously for a while in the same place to get it clean. I can only keep this up for about 5 minutes before my wrists start to hurt and I have to quit, and at the end of that time I've cleaned an area perhaps 10 cm on a side. At this rate it will take days.
There must be a better way. I'm open to chemical or mechanical suggestions, or any combination thereof.
I am not exactly sure of the material of which the tub is made - it's some sort of plastic or acrylic, or possibly fiberglass. It is definitely not porcelain, stone or enameled metal. Since I rent, I am not in a position to replace the tub with something more practical.