Some advice on "flat plating" your head or any surface. Smooth and flat are not the same thing. Glass is smooth, but not necessarily flat, think of a drinking glass. In the shop we used inhouse precision ground, thick steel plates. At home, before I used glass, I'd use thick particle board. That super heavy stuff often used for cheap furniture. It's hard, flat, smooth, cheap and holds the sandpaper well. 1" or two 3/4"s sandwiched. Clamp the paper to the board.
When you first start (220 grit wet and dry) put a couple fingers right in the middle and with very little pressure, do a couple of figure eights. That should show you any extreme gouges or warpage.
Ideally you should place a weight on the head then use your thumb and two fingers on the sides to push the head in circles and figure eights, alternating rotation. It may not make sense, but it will give you a much flatter surface. Placing your palm on the head and pushing it back and forth will give you a convex surface. Resist the temptation to push harder and go faster. Doing a good job can be slow and frustrating. And sure, guys will do that and say it worked fine, but so will just bolting your head back on with a new gasket....for a while. Sanding on a flat plate will always have tendency to cut more on the outside periphery and you have to mitigate for that as much as possible.
Spray your paper with brake cleaner often and blow it off then spray again before more sanding. That clears the paper and it well cut much faster. One sheet will usually be enough unless your tear it. The grit will get less as go for a bit smoother surface at the end. And you don't want a buttery smooth finish. A certain amount of "roughness" holds the gasket better, that's one reason why milling is the best choice. I am a professional at resurfacing, but I'm also an extreme DIY 'er and many things can be done well at the home shop with some basics tools, skills and patience.
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