After a few feeble attempts at using a belt sander and a hand plane ( as if that was going to work) I decided to build a router sled. I don’t have much in the ways of a work surface that can accommodate a piece this size so I fashioned up a few sawhorses, a bed to hold the slab, and then the router sled itself. The whole thing took me an afternoon and is made of MDF.
Because the slab varies in height so much I wanted a easy way to adjust the sled potion without having to move the walls of the bed. So the sled is attached to two guides each with tracks to hold the sled and led it slide up and down. I can go from 2-1/2” clearance to about 7-1/2” (if I ever decide to work with more uneven thick materials), and can accommodate a piece up to 3’ wide.
Its a little wobbly, so I can think of a few ways to stabilize, and leveling the whole thing is still a chore, but overall i’m pretty happy with the set up. After going down the whole length I can pretty quickly hand plane off the score marks between passes. (The picture shows progress on the more flat side being worked on, the hard part is yet to come.