Walls and Ceilings - Knockdown Tool?

Is there a special tool to use for knocking down splattered mud, or are standard taping knives the things to use?

(I had to pour a concrete slab (2 feet by 3 feet) outside once and used a standard trowel to try and get it nice and smooth. Only a few days later did I discover there are special trowels that are slightly convex for that purpose.)

I have an old 12" drywall knife I use - it helps if the edges are slightly rounded. Some of the pros use a very wide knife [similiar to spray shield, maybe the same]

Mark, once again, is right on the mark. I use a 15 inch blade - and sometimes wish the corners were a bit worn as he mentioned. However, it works great! I find that it is more about timing than equipment for this project. If you begin "knocking down" to soon - you end up with streaks that show. To late, and you get little of the knockdown effect. Every wall/ceiling is different, depending on the finish under the splatter. Some gets workable in a couple of minutes - some (less absorbant finish) take up to half an hour before you can start your finishing work.

The best tool I have found is called a magic trowel. It is 18 inches and looks like a squeegie. Can find it at some paint stores.