Vinegar and a pumice stone will handle calcium scale.

Yes, but with care. For light scale, a 50/50 white vinegar and water spray does the trick. Spray it on, let it sit five minutes, scrub with a soft brush, rinse. For stubborn buildup, a pumice stone (wet it first) will gently grind it off without scratching glazed tile or porcelain. Just don’t go nuts — anything unglazed (natural stone, some plaster) will get wrecked by both vinegar and pumice.

Plaster is the tricky one. Acid eats it, and abrasives gouge it. If it’s a plaster pool or plaster wall, skip the vinegar. Use a non-acid calcium remover (sulfamic acid based) or just elbow grease with a stiff nylon brush. CLR is overkill and too harsh for many surfaces.

Don’t use steel wool. Don’t use bleach. Bleach doesn’t dissolve calcium, it just makes it white. You’re not cleaning, you’re cosmetic-ing.

If it’s on tile, vinegar is cheap and effective. If it’s on plaster, go easy — you’re better off preventing scale with a water softener than scrubbing it off.