I would assume stuff like the big skinny flat ones in the middle of the pic for "slopes"
http://www.synrockholds.com/newscrew.html

I am considering building a crack machine to put up at the local gym to help work on some basic crack climbing technique as our crags don't offer much crack climbing.

The crack would be 12' - 15' high and vertical. It must be fully adjustable from offwith to tips. The key is I would like it to be adjustable fast (i.e. a climber does a lap with offwith then goes to a lap of tips then a lap of fists, and so on). Consequently the whole bolt / All Thread idea most people use wont work as that takes too long to adjust. So I think I may mount the adjustable side of the crack to a sheet of wood. The sheet of wood will be mounted to two or three medium duty sliding drawer tracks which will then be mounted to the climbing wall. But the question is how can I lock the sliding side of the crack into place sense it will be on a track?

Once built should I line the crack with some type of grit? Previous searches shows about 50/50 with some saying its a bad idea as it will rip your hands up and the other half saying its important to do such as you cannot get any friction for the feet without it. How about using fine grit, fine enough to provide some friction but not rip the hands to hell. Is that a good idea?

Is it even worth building, will it help teach any technique at all? My idea was to read a book about crack climbing then go and practice the locks and techniques taught in the book on the crack machine. Is this a reasonable method of learning before I take a trip to Yosemite and actually try some cracks?

For the space that an adjustable crack machine will take up, you could easily build three or four different sized hand cracks. Fingers, off finger/tight hands, perfect hands. On the small sized cracks, bust out the shaper or sander to create tapers and a variety of sizes throughout the machine.

If the wall is only going to be 12' high, I would also suggest a slight overhang (maybe 5 degrees).

Are you already drunk?

While it is the case that to learn technique, you don't want to have to use your muscles, learning crack technique (like any *single* technique) doesn't take much time. In the long run, you'll be better served to keep it smooth, and parallel, so when you really need that power reserve, in muscles that are impossible to train for otherwise, you'll have it. Otherwise, you'll spend all this time and effort building the stupid thing, get 30 hours of use out of it, then switch back to whatever power-endurance training you were already doing.

The idea is that when the crux is enduro ring locks with featureless stone, you've got the muscles necessary to punch it alongside the technique to actually hold on. Yes, it makes the feet much, MUCH harder. Welcome to crack climbing in the real world. If it was heavily featured, you wouldn't be training for it.

Also, vertical cracks are not nearly as useful as overhanging cracks for training purposes.

Even so, 2 weeks in Indian Creek and another 2 weeks bouldering at Vedauwoo will teach you more about crack climbing, especially hard and steep crack climbing, than any crack machine.

As for coating it, do you have any insight on what to use to help weather-proof it AND keep it smooth? Should I just use paint or is there something better to use? I want to do this all in one shot so I'd rather not experiment if I don't have to.

I'll be using the regular SPF framing lumber, since it's most economical. It is going to be outside. I was going to make it permanent, but my room mate convinced me to make it "portable". Basically it's gonna look identical to the picture I posted, but the stands won't be permanent. We'll be able to break it down and stow it aside, covered in the back yard when not using it to try to keep it away from the elements, but we live in WA and it rains all the time so we'll see how that goes, haha!

And even when it is raining, I think if we just throw a tarp over it we can just climb inside it like a tent.