Natural edged slab wood can be turned into some amazing projects. But taming the rough slab can be a chore. A planer needs a jointed surface on one face of the plank before it can do its job. A jointer works great for this, but a typical slab will exceed the capacity of most jointers. A planer does one thing, it makes boards thinner. Unlike a jointer, it cannot straighten a board. A common work around for this scenario is to build a sled for you planer, set the slab on the sled and shim it up so it won’t rock. I find the approach time consuming and frustrating. The shims never want to stay put so securing the slab is iffy at best. And I’m not a fan of using screws to fasten the plank and then passing it through the planer. I prefer to use good old fashioned winding sticks to find and mark the high spots then use an electric hand planer to joint one side before sending it through the planer.
Winding Sticks Catch The High Spots. Sight across the tops of two winding sticks to find the high spots on the slab. Use contrasting woods for each stick. Pick a flat spot as your reference. Slide the second stick along the length of the board marking the high spots as you go.
Chalk marks the high spot. Mark the high areas to be removed. Chalk is fast, easy and visible.
Joint The Board With An Electric Hand Plane. Remove the high spots with an electric hand plane. Check your work with the winding sticks or turn the slab over on a known flat surface and check for rocking or gaps. Continue until the face is reasonably flat and the slab won’t rock.
Plane The Rough Side Smooth. With one face reasonably true, go ahead and plane the other surface smooth. Take light passes until the whole board is surfaced. With the top face smooth and flat, thanks to the planer, flip the board over and surface the other side.
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You didn't answer my question. I want to joint a 8" board on my 6" jointer. George Vondriska demonstrated a technique to do so a while I ago and I have forgotten it.