What it does: removes material very slowly from the entire interior surface (not just the parts you want, or casting flash and machining errors).
Any area that isn't clean won't get ported.
If you know where you want it ported, mask the other areas with something acid-resistant such as wax. Don't melt wax into the manifold, it won't come off the areas to be ported by scraping. It may get hot enough to melt the wax anyway.
Chemical: for cast-iron, most strong acids will work but very dangerous to handle. Aluminum, you can use lye or acid, but they work much faster. Never add water to acid - you add acid to water.
Put it outside, plug the port runners, leave the flange open and fill with acid. It probably affects the bottom slightly more, as bubbles will tend to follow the walls upward.
Best results: plug everything, and rotate it 90° every 15 minutes for several hours, drain, examine.
Wear rubber gloves, goggles, respirator (not just a mask). Tell someone to call you back, and call 9-11 if you don't answer. Throw away whatever you wore.