I just got a used Stanley #7 plane from eBay, these planes are also known as jointer planes. With their long base, they can make a very straight edge in wood, or flatten a large piece of wood. In my case, I’ll be using it to flatten my work bench top. From my research, this is a Type 15 plane, made in either 1931 or 1932. Buying a used plane I think saved me some money. The newer Stanley planes aren’t rated high on Amazon, and have plastic handles. Some say they take a lot of work to flatten and get everything right, as Stanley isn’t producing a quality product any more. Apparently the older ones were better, so I decided to go that way. There are other very nice planes you can get from Lie Nielsen or a Veritas from Lee Valley Tools. The prices for those higher end planes are a lot more.
Both wooden handles seem in great shape. I’d say there was some rust that someone cleaned off. There were a couple areas on the bottom where a tiny lump of metal was protruding, and would scratch whatever I was smoothing. The blade had lots of small dings in it, and seemed to be sharpened at a very shallow angle.
You start by using a piece of plywood with one good edge. Either the factory edge that was cut when you bought it, or one you cut with your table saw. If you don’t want to make marks on the plywood, tape a piece of paper to it, and mark on the paper. Lay the short edge of the square along the plywood, draw a mark the entire length of the square. Flip the square over, and move it about 1/16″ of an inch away from the other line, and draw a second line. These lines should be parallel. You will see from the video above, that for a framing square, you can use a center punch and if your square is less than 90 degrees, hit the inside corner, if your square is more than 90 degrees, hit the outside corner with the center punch (hard), on a metal vise. This spreads the metal out ever so slightly and will move the angle of your square slightly. Be sure and check it again on the plywood, again making two marks with the pencil. See if you need to do it again. You can make several punches in a small area to spread the metal more and more if needed. Make sure and don’t go too far. Also don’t get too close to the edge, or you may mess up the straight edge, and you’d need to file that down.