The only way i could ever make it work is to mark all four sides and plane at an angle to each line. Going lightly to stop at or before it a bit.
Then when all edges are to the line go across or diagonally keeping the plane level.
Mark the board with pencil lines every few inches and if you plane a line off on one of the slopes remark it.
The hardest part for me is to bAck off the iron and sneak up on flat with the plane.
Go slowly and watch the lines. They get shorter and shorter

This may not be your issue, but I wanted to add that if, in following Collin’s advice, you encounter strange planing behaviour where the plane will skip over identified highspots, and take a shaving on low spots, the board might be twisted.
Checking for twist, even on the opposite side of the face, is valuable, because a twisted board is deeply unintuitive to read.
Several times, i’ve sighted a board, and checked with a straightedge, only to have the plane make the low spots lower, and the high spots highler.
Most of those times, it was because twist made me misread where the high spots actually were.

I’ve come to see winding sticks not as optional for stock preparation, like a straight edge is, but as completely essential.
Unlike the square, the winding sticks don’t have to be perfect in my experience. They just need to get you get you close enough that the readings from your other tools make sense.

Might not be your issue at all, but worth keeping in mind.

If you keep running into trouble, stay patient with yourself. Maybe practise stock prep, not for a particular project, but just to practise stock prep.

EDIT:
I’ve also found that using long planes have more of a tendency to ride over a big high spot than shorter planes.
It’s easy to just skit the the sole over them, keeping the iron in the air. Using a shorter plane, or skewing the plane, helps this.

• This reply was modified 1 year, 9 months ago by Sigurt Dinesen.

One thing to keep in mind is that not all situations require 2 perfectly flat and coplanar faces. A small variation in thickness may not be noticeable. For example, the underside of a bookshelf. Also, the exact thickness of a piece is not always critical; it’s probably not going to be noticeable if that same bookshelf is 3/4″, 23/32″, or even 11/16″, as you should mate the individual dados to the individual shelf pieces anyway.

Also, using a smaller plane (#4 or #5) and focusing on the high spots alone while checking for twist is a good path, as noted by others here.

I think georgewall4 and deanbecker’s point is generally undervalued, especially in this branch of the woodworking community.
For example, Paul Sellers often says that e.g. plane sole flatness is not as important as people make it out to be, but no one seems comfortable stating the tolerances for the work they produce.

In one of his Mortise & Tenon magazine articles, Klein wrote that aprons/rails on pre-industrial tables might not even be planed at all, but just left rough from the saw mill.
I think he actually performed a (very small) study of the precision of pre-industrial cabinets..

(In fairness to Sellers, he usually shows us with his square etc, but either it doesn’t show on camera, or he is literally dead flat when he says he is)

Edit: Found the Klein article. He claims surfaces in pre-industrial work has an average of 0.03 inch variation, although outliers are frequent and extreme.

• This reply was modified 1 year, 9 months ago by Sigurt Dinesen.
• This reply was modified 1 year, 9 months ago by Sigurt Dinesen.
• This reply was modified 1 year, 9 months ago by Sigurt Dinesen.
• This reply was modified 1 year, 9 months ago by Sigurt Dinesen.
• This reply was modified 1 year, 9 months ago by Sigurt Dinesen.

Are you getting a hump in the middle along the length of the board? This can come from applying extra pressure to start the cut and from applying extra pressure as you come off the board without realizing it. It is a useful exercise to let go of the knob completely before the nose reaches the end of the board and just push with the tote to convince yourself that you don’t need to push down to continue the shaving. Then, learn to deliberately unweight the front of the plane as you come to the end of the board. A different grip on the knob can help, too. See, for example, https://commonwoodworking.com/wp-content/uploads/2018/02/P1000047-1.jpg . Or, rotate the front hand so that the pinky is on top of the knob and the thumb and forefinger are pinching behind the knob (probably on the cheeks). Your elbow will likely be up in the air. This is an “old fashioned” grip you see in old photos sometimes. I find it reduces the tendency to push down at the end of the stroke.

Extra pressure entering the board could be a habit, but could also be a sharpening issue.

All this being said, even when your skills improve, there’s still a tendency to plane a hump into the length of a board. So, it pays to learn how to form a hollow. Simply put the plane down such that the mouth is about 1/4 of the way back from the middle of the board and push forward until the mouth is about 1/4 of the way ahead of the middle of the board and lift the heel of the plane as you push forward. This lift will cause the iron to come out of the wood and nip off the shaving. Repeat. After a few strokes like this, the plane will stop cutting because you’ve cut a hollow into the center of the board. It only runs for a short section of the board, though. You can extend the hollow by extending the cut a but back and a bit further forward progressively.

If you play with this and get a feeling for how it works, you can start using this to counteract the tendency to plane a hump into the length of the board. Periodically make hollows in the middle, then return to working the length. You will find, when you go back to working the length, that you get a shaving at the start of the cut, then no shaving where you have the hollow, then a shaving on the other side of the hollow. As you work down, you will get a full length shaving. At that point, you will likely start making a hump again. If you have more thickness to take off, make another hollow and repeat. Don’t read this as a recipe that says you make hollows and then take off the ends repeatedly. I’m just trying to explain another pattern of planing you can use to monitor the state of the board and to adjust how the work is progressing.

If I want dead flat, one of the last things I do is to reduce the plane set to a fine, fine shaving, put a hollow in the middle, extend it, and then work end to end until I get a continuous, fine shaving. I then stop. I can’t get flatter than that.

As an aside, the more I plane, the more I find that I work on the bench top against a stop rather than in the vise. It is faster for me and the board is fully supported. It eats up bench space, but my bench is long enough and I now consider this a requirement.

• This reply was modified 1 year, 9 months ago by Ed.
• This reply was modified 1 year, 9 months ago by Ed.