How do you seal a crack in a concrete pool that hasn't been plastered?

Question:The pool shell has a hairline crack and is ready to be plastered. I need something that won't be too slick, but not too abrasive.

Answers:
Quickcrete makes a product called Water Stop or you can use Hydraulic Cement. These are a powder that you mix with water and apply with a putty knife or trowel. Being it has cement in it, plaster will adhere to it. There is another called Aqua Plug but I'm not sure who makes it.