The common wisdom to flatten raw stock, is to first plane a face flat on a jointer. To get to opposite face paralleled and flat, you run that newly flattened side face down in a planer to your desired thickness. Sounds familiar, I’m sure. Hard to do that with 8” stock when you have a 6” jointer though.
The common wisdom also states that if you just try to run that raw stock through a planer, flipping it each time until you get it flat on both sides, you’ll end up with anything but. Supposedly, the planer rollers will flatten the board temporarily while it takes off stock…so what you end up with is a smiley-face shape board.
I’m currently breaking down some thick raw stock – about 5/4 oak, 8 inches wide. I only have a 6” jointer, so I can’t go the traditional route of jointing, then planing.
Thinking back to the conventional wisdom, I found it difficult to believe that rollers in a small planer had enough strength to flatten a board that thick.
So, I put a piece of this 5/4 raw oak through the planer, skipping the jointer. I put it in so that it would like like a frown, if viewed from the side, with a high point being planed off first. When I finally planed it down enough to reveal smooth wood on that side, I took it out, and a straight edge told me it was dead flat!
Flipped it over, planed down the other side….and now i have two flat, parallel sides!
Having said that, I think conventional wisdom would be true for thin stock, or soft wood.
My neighbor, an older man who makes great stuff, said he hardly ever uses his jointer, and does basically what I described to you.
-- Have a blessed day!