My wife leaves her shaving cream cans on the edge of our white fiberglass [?] garden tub, and inevitably, there are little rust spots where water reacts with the metal in the bottom of those cans.
How can these be removed most easily and inexpensively? They won’t scrub off with a brush and an abrasive cleaner, so I’m thinking there’s some sort of home remedy that might work, or something.
Thanks. And happy new year!!! 
Lime-away or CLR might remove rust stains.
To prevent future stains, go to a pet store and buy a plastic cover that fits the (very) small cans of cat food. This also fits the bottom of shaving cream cans and prevents rust stains.
CLR stands for calcium, lime, rust. It works easily, great for the shower head and toilet, and kitchen sink sprayer.
If the CLR does not work, you may need to use an acrylic polish and buff out the stains with an automotive type polisher.
There is a product I use to clean up tubs at the end of jobs, it is a polish made for the purpose and does a good job of removing minor stains and scratches and can make an old acrylic tub look new. Can’t remember the name, and having no luck googling but it comes in a pink aerosol can and can be found at the hardware store with the cleaning supplies. It or similar products can help by leaving a protective film on the tub.