The best thing to use, at least on steel is a cutoff wheel, as was stated above. Use an 1/8" cutoff wheel. Details below.
I will add the following:
When used cutoff wheels get too small for effective cutting, they make good grinding wheels when stacked together, but, for grinding within a really tight radius use a die burr, etc.
Here's my take:
For cutting, I use the .045 Sait brand wheels, for grinding, I use the 1/8" thick Sait "cutoff" wheels.
The thicker wheels last a really long time. I bet I could grind at least a ten-foot MIG-welded **** weld on 20-gauge sheet steel (if I
had to, which I would lament and would have endeavored to fabricate more and weld/grind less of course) with one four inch Sait 1/8"-thick wheel, which costs about $2.50 when bought in bulk.
Cutoff wheels should be used for cutting and not grinding if they are thinner than about 1/16", or you will get too many cuts in the metal due to too much deflection and delayed wheel reaction. This results in using the shoulder of the wheel not the nose, without grinding the actual weld which makes it difficult to grind the weld without taking parent material out of the adjacent metal or cutting into the cross section of the weld, making it thinner.
If you stack thinner wheels of a similar diameter on your arbor then grind them against the floor (if you don't have a Rock Star type floor of course) or scrap piece of steel to make them the same diameter, this can work in a pinch. Bang them enough to make the larger wheel disintegrate quickly but not enough to bend the arbor. This is a stopgap measure. When you grind, the wheels will separate slightly if above one-inch diameter or so, making them less effective than a 1/8" wheel.
There is an art to grinding a weld, and so much nuance as to tool pressure, speed, linear motion as well as angle. It is as tough to grind a weld nicely as it is to do it properly in the first place.
To me, Roloc discs are useful for about the last ten percent of metal finishing. They just deflect too much, take too much adjacent material out, don't last long enough and are too expensive to make them worth using but for the last aforementioned 10 percent.
Photo above is of wheels mentioned (the thick is on the left, thin is on the right) with my favorite Chicago Pneumatic die grinder. Alternate favorite is my Dotco.
I will say that cheap die grinders (not those above) have come a long way. Just make sure to buy good arbors that do not bend easily.
