I guess I've put this off as long as I can justify, so here
is my testimonial on scraping...
Almost all my planes are old iron ones and the soles were not flat when I acquired them. My first attempt at flattening was on a Stanley #9 1/2, a 6 inch long block plane. I taped down a piece of 400 grit SiC paper and rubbed the plane on it for about 40 minutes. After that time, it had a uniform shiny appearance so I considered myself done. I put a straightedge to the sole of the plane in several orientations and convinced myself that I had done a good job.
My next attempt was to flatten a Stanley #7 jointer plane. This plane is 22 inches long, so it is more of a challenge to flatten. I bought a couple of AlZi sanding belts, glued them down to a marble slab that I deemed "flat enough" and started rubbing. I stopped once I had a uniform shiny look to the sole of the plane. When I put a straightedge to the sole of this plane, it was not flat. It appeared convex in both directions.
Further reading on this subject at different websites made me realize that the sandpaper wears out in the middle first. So, while it is still abrading the edges of the plane sole, much less abrasion is being done to the center. This problem did not occur with the block plane since the sandpaper sheet was much larger than the sole of the plane and I did not rub it on the same linear strip.
After some headscratching, I went back to the web for another search on flattening methods. I found BugBear's treatise on scraping and related links. I bought some 10" square doublecut files, a Sandvik carbide scraper, and some prussian blue and went at it.
I put a dab of the prussian blue on my marble slab and spread it with an ink roller or "brayer". When I had an even blue tint on the slab, I placed the plane on the slab. I found that I had to wiggle it a little in order to pick up much blue on the sole of the plane. If I just plopped it down and picked it up, there would only be a few specks of blue.
I lined the jaws of my vise with tape so I wouldn't scratch the sides of the plane and clamped the plane in the vise over the cheeks of the plane. The file was surprisingly aggressive in removing metal. It only took a few seconds to remove the blue and some of the underlying metal. I wiped the sole of the plane with a rag to avoid transferring filings back to the slab and placed it back onto the blue slab. Each cycle only took about two minutes. In about 40 minutes I was getting blue over most of the sole of the plane. I changed to the scraper to smooth things out, and after about 10 iterations I considered myself done. I resmoothed the prussian blue every other iteration with the brayer, and had to add more blue about every 10th iteration.
This plane definitely performed better after flattening than it did before. Not as much chatter and just a much more positive feel when the blade bit into the wood. I have found that most of my older planes have a concave sole, where the mouth of the plane is suspended over the surface to be planed.
I have flattened several more plane soles using this method since and have gotten much quicker at it. I have learned that I can be more aggressive in the early stages of flattening, taking off more material with the file before going back to the slab for another blue application.
I don't bother to polish the sole of my plane with fine grit sandpaper. I wax the sole with an old candle and figure that the wax fills in any scratches anyway.
The filing and scraping method of flattening a plane sole has worked much better for me than lapping on sandpaper. The sole comes out flatter and the progress towards this goal is much more predictable. Once a bit of experience is gained, filing is much quicker than lapping on sandpaper.
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