Well this is a subject that I have some experience in but the best advice is "your on your own". There is no one method for restoration that works fine every time. I always attempt to do some cosmetic work on parts if they are in need. Now as far as using sandpaper to clean up a 27.2 seatpost, that will work. You have to do some pretty heavy sanding to bring it down to 27.0. I assume that you are doing an aluminum post. I would use a very very fine sandpaper and lightly sand in the long direction. If you are trying to remove zig zag patterns from it, you can start with 400 grit then move to 600 and finally you can use a chrome polish to buff it to an almost new look. Alloy parts can be made to look good so long as you are careful not to remove a lot of material. Another trick, you can use baking soda on aluminum to bring it back to life. Put some on a wet paper towel and buff it into the aluminum. When you are satisfied with the results then wash off the baking soda completely. As for steel, if 0000 steel wool does not bring it back then it is a trip to the plating shop but find a good one. There are shops that can plate an item so it does not look like it was dipped with a lot of build up. The old Schwinn collectors are always looking for a good chrome plater that can do rims and not ruin the knurling that is on the face of the rim. That other tried and true method of ignoring some flaws always works best. Some of us think it brings character to a used bike.
Ray Homiski
Elizabeth, NJ