We all make choices of how much work we want to put into our trucks. For me, hours of lapping with the intent to better the head gasket situation was not that much of a task.
But as my old director would say to me, you're willing to do a lot more than others in your situation. One of those times was instrumenting a Waste Management garbage truck in the middle of summer in Atlanta because as the department manager, I couldn't ask my employees to do it by themselves if I wasn't. It washes off.
Ed is just finding a way to make money; they rape and pilage on things like this.
You can do it or not. People have been fine not lapping the decks.
150 then 180. You're done.
New paper, top-down, left to right and back.
New paper, top-down 45º angle, left to right and back.
New paper, top-down -45º angle. left to right and back.
New paper, left-right, top to bottom, and back.
You have to keep changing paper because the hard iron surface destroys the grit. Otherwise, it takes longer. Sandpaper is cheap.
Depending on the surface and cleanliness, repeat.
When the surface starts to look good, switch to 180 grit.
You need a flat plate or object. Some guys have run a piece of hard lumber through a plane until it "sparks out," measured flat. I've bought aluminum and steel cutoffs from my metal supplier for a few dollars that measured flat with the straight edge I took with me.
You can buy an aluminum plate from McMaster and check it for flatness. If not flat, they take anything that's not been used back, like Amazon.
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Amazon has lapping plates; this one says it's flat to 0.0005", like mine.
Amazon.com: DFM Diamond Lapping Plate MADE IN USA (220 Grit) for Flattening Waterstones and Sharpening : Tools & Home Improvement
They have other stone flattening tools that might be as flat. You have to measure.
Here's one that says it's 150 grit; I'm surprised. But I wouldn't use it bare; I'd use the paper, and this would be the holder. But, again, it needs to be checked for flatness.
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There are different ways of doing this; you need to get imaginative.