On some games the stainless steel metal ramps are grained rather than polished. While there's a quick and easy method for ball guides using an abrasive ball (see this article), this approach won't work on a u-shaped ramp.
Instead you can typicall do the bottom part of the u-shape using a nylon or wire abrasive wheel (I prefer the nylon). The wheel should be held perpendicular to the ramp and rotate in the same direction as the grain. Start out with a medium grit like 180, if necessary, and move on to a fine grit like 400.
For the inside sides of the ramp I use an old worn nylon abrasive wheel that's only about an inch in diameter. You could also use a Dremel Tool with a wire wheel, although it will take a while.
The final choice is sandpaper (I prefer drywall sandpaper). Start out with 180/220 grit sandpaper working in the same direction as the grain. Do not move in a circular motion. On some games this will match the original finish perfectly. If not move on to 400 grit sandpaper using the same method.
For the outside of the ramp you can use an abrasive ball or wheel, although the ball is easier to work with.