Repairing bathtub chip

While replacing a window, the installer dropped something on the top edge of the tub, causing a chip (but no dents) about 3/8" diameter. His boss is going to contact someone who repairs such chips by spraying the entire tub with something.
Any comments or suggestions for best/worst materials to use? How about touchup paints in small bottles with built-in brushes? Because the chip is on the edge, the inner shower curtain prevents it from getting wet. The tub is some kind of a plastic, not cast iron.
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