here we see what happens when you take an aluminum head to a traditional machine shop for skimming or "cleaning" as part of head gasket work.
in this case, the black gouge marks are shown where i stained a scrap head that i knew had been skimmed, then used wet and dry paper to cut off the top layer leaving the gouges in relief.
you will *never* get this head to seal properly. and unfortunately, this kind of damage is pretty much *inevitable* from any traditional skimming operation. and what's worse, unless it's really bad and you know what to look for, you can't see it /unless/ you do what i did.
for the techs that will want to say, "but i get heads skimmed all the time and never see a problem", i will say "yes, /you/ don't see any problem because /you/ will never see that car again because the owner is so pissed at spending all that money only to have it fail again a couple of months later, they'll either throw the car away or blame you and have someone else have another go".
anyway, this happens because the cutting tip either picks up grit on the hole side left over from the head casting operation, or the swarf from the prior cut folds under and drags. both gouge rather than cut through the soft aluminum. the only way to re-finish an aluminum head successfully is by flat bed grinding. very few shops are equipped to do this.
the best method of dealing with head prep for shop work is to use a thin bladed scraper like a razor blade [which doesn't gouge] and plenty of gasket remover. do NOT use abrasives unless on a flat bed, and only if you know how to clean the thing properly to remove embedded grit. do NOT use scotchbrite or anything of that nature with spicular alumina in it. guaranteed to cause premature wear in the whole engine since it's so hard to remove.
[yes, there all kinds of other condition issues with the head - that's why it's scrap.]