Almost certainly a warped head, but some of the methods being suggested here to flatten it are a good way to ruin a perfectly good head. Do not use sand paper or concrete both are far to coarse and inaccurate. The best way is to use fine valve grinding paste on a piece of glass, preferably 1/4" or thicker as that's flatter. Do not use a scrap double glazed panel as they're often slightly concave. oil the glass, then smear on some fine valve grinding paste. Place the head on it and move in a figure of eight motion, every dozen or so figures of eight stop, turn the head through 90 degrees and continue. Every so often lift up the head and check the lapped surface, low spots will be obvious. Continue, adding more grinding paste as necessary, until the whole surface is an even mat colour. Wash off, then refit using a new head gasket and torque down gradually, ie bring the bolts up together, do not torque down one fully then the next and so on. If the order is not known tightening diagonally is a good rule of thumb.
One other thing to check is to make sure the bolts (or one possibly) aren't bottoming out too early. Without the head try fitting them, and make sure when fully inserted the gaps between the block and the head of the bolt are all less than the thickness of the head. If they are bottoming out, even when torqued down they won't be squeezing the gasket correctly.
You would literally never get done doing it this way..
Well, you would stop but in the end, it wouldn't be much different from just leaving alone and wiping it off with a rag after you scraped it in the first place.
These heads are so warped and have such large dips and valleys in them that you're never going to accomplish anything that way.
There is absolutely no problem with laying them on a fairly flat surface, with sandpaper on that surface and working them in all directions as you turn the head around a quarter turn multiple times so this way you're making up for any possible surface of regularities.
I prefer to start with 80 grit but I'm guessing you're going to say that's too course but it works great for me. Sometimes I'll go over it afterwards with something a little bit finer but nothing finer than 120 because this doesn't need to be polished to a smooth surface and you'd be amazed how pretty and smooth they look after doing 500 to 600 strokes with the 80 to 120 grit that I do.
Yes, it takes about 500 strokes doing it this way and I put a good deal of pressure down on the head to get it to where you have actually sanded the whole surface so there's no more high spots holding the rest of the head up off of the sandpaper.
They are that bad. And this is probably why so many people have repeat problems with head gasket failures despite the fact they claim their twerking them in the correct pattern and to the proper specs.
This is not rocket science and it's not very precise either. They are still very forgiving and it does not need any certain type of machine slash engine builders and finish on it to work perfectly.
These engines are actually junky little low performance turds that just luckily are pretty forgiving of most of the abuse, neglect, and design flaws, and low quality parts they're made with.
Sometimes though you have to take matters into your own hand to make a head relatively flat and smooth versus the abomination it was when you removed it.
So sandpaper away I say. It's worked well for me and I've done this more than most human beings alive.
The average person or the majority of people in this group probably haven't done over five in their life and maybe not even over three.
I really don't see that as a large enough sample to prove anything.