End Grain Cutting Board

I'm a fine-grained sand cutting board, and of course its very important I get the flat surfaces. Because its fine grain, I can not run it through my planer, so I'm working with a belt sander and a random orbital sander, but I can not get a flat surface. Is there a trick to this? I always seem to have an oscillation on the corners when I put the board on a flat surface.

Almost need a sander, table and a 150 grit belt. Then sand lightly with an orbital with 120 grit. Caution DO NOT use the edges of the orbit, the use of the medium.

When the fine-grain planing, the plane iron should be the other way, which can be done with a meal plan. If you have a plan for Jack, remove iron and return it so that the bevel is higher. And obviously, not too much "bite" on it.

Use a sanding drum on your drill press.

When absolutely necessary to have fine grain exposed, as you say, it can be sanded flush by attaching two parallel strips of wood (3 "-4" wide and 3-4 "beyond the end of the cutting board) along each side, slightly below the line of existing board.
The wider strips will hold the sander in a seesaw, it extends beyond the end plate will cut full length "dressing" of the commission.
Note grained exposed is not an ideal method of finishing.
It leaves large areas for liquids to penetrate and swell the region, which will eventually overload the timber and / or carpentry, causing failure.
A much better method (if you have the tools, time and talent) would have a cap piece dovetail, which would resolve many issues, and last much longer.