Stair crack repair

I have 5 double hairline cracks (each 1-1.5 inches in length) in my pool stairs, probably from the ground heaving/settling after the pool was put in. The cracks are at the top next to the concrete so they are not underwater. How can I repair these as I don't want them to eventually spread down further and into the part of the stairs that is in the water. I was wondering if there is some sort of material like epoxy or something that I could push into the cracks to possibly stop further spreading. It would preferably need to be white.

Thanks

If the cracks are in plaster, you can get plaster repair form your pool store. It's a powder that you mix with water. It works pretty well in bigger cracks, even applying under water, although more time consuming underwater. I'm not sure how well it will work with hirline cracks as there may not be enough of a gap fore it to adhere in. Worth a shot though.

Thanks for the suggestion, but the stairs are plastic or some fiberglass/plastic combination.

Cheers