Hi Mo,
There are several methods for this in museum restoration work, all are extremely arduous and best. I'm with Timbeal, scrub across grain with share blade and call it good. This is the fastest solution.
Have had to plane some members to flush out. Too little to gain.
In the future, if you have to maintain a certain finish effect or presentation, switch from edge rule to line rule and you will not have to true your timber, (flush out?) in anyway.
Best of luck,
jay