Acid etching holds
Synrock holds are composed mostly of sand grains held together with a limestone-like binder that is slightly acid soluble.

When you acid-etch synrock you expose the sand grains on the surface of the hold. The result? Sandstone.

Cover your hold with warm water and add a few ounces of Muriatic Acid. Let soak for about a minute. For large holds like the big pill, you can do 1/2 the hold at a time with the bucket about 1/3 full. If you are going to touch the acid, wear rubber gloves
Rinse hold off and brush with a green scrubbing pad to bring out the sand grains. Repeat as many times as needed. The more you repeat the process, the rougher the hold texture.
Muriatic acid is sold in most large hardware stores like Lowes or Home Depot. A gallon costs around $3 and is enough to acid etch hundreds of holds.

This stuff isn't "movie acid" in that it won't burn a hole in your skin if you get any on you. However you should definitely wear rubber gloves.

Some Climbing gyms soak plastic holds in muriatic acid overnight to remove chalk build-up. Do not leave synrock holds in acid for an extended time.