Yes you can seal it, several things to consider.
If it is the precast that is similar to what is used on new home construction then you should get out the yellow pages and call some stone casters or supply companies in your area. I have Noble cast and it also stained, but most of the stains will, for the most part eventually wear off. Rust is the worst to get off. You can use a light bleach solution, but not acid. Anything more than 10-15% Acid will eat the stone up. Experience talking here !. Also you should wash off the coping after you swim if you use a SWG (Chlorine Generator). The Salt does penetrate the stone. I did not seal my coping very well last year but plan to do so this spring when the temps get back up to normal. I have 248 Linear feet of precast coping since I used it on all my steps and decking as well. You should use the same sealer each time you treat it. The sealer is not cheap, I pay about $30.00 per gallon from a wholesale stone supply. Areas that were treated well do not stain deeply. Areas that were no treated do.