Re: Greasless compounds vs sandpaper
Dave,
When I sand metal I try to analyze whats happening. If pits come up in places they weren't before, I figure there are pits (porosity?) in the casting and I am exposing them as I grind down into it. But if they are the same old pits, then I know I just haven't gotten them yet.
As for scratches: I look and see what size they are. After some experience I am able to tell an 80 grit scratch from a 150 grit scratch, and so on, and that tells me which sandpaper I didn't use long enough. If I see what I think are 150 grit scratches, then I know I need to go back and use the 220 or 240 some more.
In your photographs I couldn't see the scratches, but I did see the pits.
One thing you can try is using scotch brite (surface conditioning) pads. They seem to move the metal around some, rather than just grinding it off. They seem to fill in scratches and pits with the adjacent metal, kind of like spreading butter on toast. I use them on a die grinder at high speed, (probably take the rest of your life to do it by hand), sometimes with water to keep the metal cool if it is sheet metal. (Heat shrinks metal.)
Richard
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