This is a similar question about smoothing an iron I did for the "Nopistons" list. This is for irons that have been run so long they have developed a chrome like finish. New seals will take a year to seat in on a chrome finish. Same as honing a cylinder in a piston engine. I prefer the better oil holding of a deeper finish to that of a stock Mazda. But almost anything will work. 

 

Also Yawpower has the web site back up, and I urge you to read all of his tech articles about rotaries. 

 

 

You can lap the surface yourself and you can get it ready with a random orbital sander. Used dry with 180 grit paper,
this alone will get most of the surface close to one color. You can run the front iron against the front side of the center iron with oil and valve grinding compound in between. Wipe it off now and then to be sure all of the surface
has a flat finish with no shiny spots. This will produce a very satisfactory result on both irons. About half way through, rotate the top iron 180 degrees so the ports are not together the whole time. If you have a mill or access to one make a bar and plate with holes in the plate to mount a bearing on a bolt. Plant the bearing in the center hole, so it impinges the iron about one inch. Run the mill slowly and keep adding grinding paste as you go along.
A few layers of Visqueen under the bottom plate will keep the mill clean and allow reuse of the paste. Bolt on a rod of some kind and run the rod through a fixed bracket or just any bracket on the bed to keep the iron from spinning. Then you flip the center iron over and do the same again with the rear iron. If you drill press has an electronic speed control that allows for very slow rotation, you can use it as well.

This method costs nearly nothing and produces very flat irons with a nice oil holding finish. Clean the irons at the car wash with high pressure water and soap. Spray with WD-40.

You can do this by hand as well, but it will take all day long. This can be done quite a few times without bothering the nitride. Piece of cake. This has to do with finish, and is not much help with wear grooves since it removes very little material.

Lynn E. Hanover