To restore the shine to an acrylic sink you can dilute some bleach using a ratio of about 1 tbsp. to 1/4 cup water. Dip a cloth in the bleach solution and press it over the spot like a poultice for a few minutes.
Or you can buy a bathtub refinishing kit that works on fiberglass, porcelain or even the acrylic sinks, tubs etc.
It's a bit expensive but well worth it.
Yes you can refinish the shine on the fiberglass tub using a DIY fiberglass tub refinishing kit.
The fiberglass tub refinishing kit is around $135.00 but does work great and is cheaper than installing a new fiberglass tub.
You can also polish the fiberglass tub with some fiberglass polish.
To get scratches out of a fiberglass tub you can wet sand the fiberglass tub.
Then apply some fiberglass scratch repair or fiberglass epoxy to the fiberglass tub where the scratch is and then wet sand the epoxy after it dries.
You can also buff the scratch on the fiberglass tub with some petroleum jelly which sometimes helps get rid of slight scratches in the fiberglass tubs.
You can then also polish the fiberglass tub with some fiberglass polish.
Scratches can sometimes be buffed out of fiberglass as long as the scratches are not too deep in the fiberglass.
For scratches in the fiberglass that don't penetrate down into the layers of the fiberglass they can usually be wet sanded and buffed out.
Deeper scratches will require and epoxy and then be wet sanded to get rid of the deep scratches in the fiberglass.
Before filling in a deep scratch in the fiberglass wet sand the deep scratch to smooth the outer surface and to give the epoxy a rough surface to adhere too.
After sanding the surface of the fiberglass put the epoxy on in small thin layers and then after the fiberglass repair epoxy dries then you can wet sand it.