Well, the board has been planed down to the line in three evenings, about four hours total I guess. I was pretty pleased with myself and got my straightedge to see how straight the board was. Remember, the top face was flattened first, and the thickness of the board marked with a marking gauge from the top. If I remove all wood to the line all around the board, the bottom side should be rather flat too. So you can probably understand how perplexed I was to see a considerable bow in the wood. Quickly turning the board over I found the formerly very straight face to be not very straight anymore.
The wood cheated me!
Several things worked against me. First we had a change in the weather. The long, cold and wet spring changed into nice sunny summer weather. Then, I have removed more wood from one side then the other, which tends to release tension in the wood in uncontrollable ways. And finally, the worst bow is at the end with the two big knots. Such defective wood can do anything you don't want, of course.
Anyway, these are times when you only want to stick the project in the stove and have a big bonfire. But for now I have just stickered the wood and clamped to workbench top, hoping a little bit of the bow will reverse again. The rest will hopefully straighten out when screwed to the the undercarriage.