When you profile a knife, you are grinding against a sharp edge. The sharp edge ripps the grit off of the belt and wears them down quickly. Many makers use worn (but still good) belts for this task. I profile on the contact wheel but either the contact wheel or the platten will work.

As for your question on; "What are the different grits for?"

Grit is the term used to identify the courseness of the belt or sandpaper. The higher the number the smaller (finer) the grit. A small number like 36 means the particles are large and remove a lot of metal at once. A grit like 220 is finer and removes less metal and a 600 removes less still. Each grit will leave scratches on your steel, so you use progressively finer grits to eliminate the previous grit scratches. As you progress through the belt sequence that Don Cowels already outlined, you will have a really smooth surface at Heat Treat time. After heat treat you keep the progression going until you reach the desired effect on the steel. Many knife makers go to 1000 or even 1500 grit on their knives.

Hope this, in conjunction with Don's reply clears it up for you. If not, just ask for more info and we will help.