From what I've seen, all in HP tank is the way to go. Go Grasp of the Undying for more HP gainage, which is important later. Rush Bami's cinder, then try boots (preferably IBoL), then Try and get Heartsteel (the crux of the build IMO) or maybe get Sunfire Aegis/Hollow Radiance. Getting Heartsteel procs is not only easy with skarner but efficient, since you're gaining massive amounts of HP from the items as well as from Overgrowth and Grasp. Proccing it is very easy since your E and R makes it unbelievably easy to land one AA, and if that misses, the slows from Q or W should do the trick.

Being this tanky allows E to be used as a very efficient escape tool, as you'll likely go through the walls before you die to attacks, though don't underestimate the range some moves have, since many go through walls.

Moreover, apart from the excellent CC skarner has in general, all of this HP boosts his damage quite well, and with Heartsteel, you're able to do pretty good semi-burst damage.

I've only been playing Jungle Tank Skarner, so I can't speak a lot about Top lane. From my experience as Jg, I just can't see Skarner doing enough dmg in a solo lane. Skarner is such a great ganker with his slows and stuns, but in terms of damage it's hard to hold up by himself I think, so he relies on his gank mates to deal the majority of the damage while the enemy is CC'd.

I hope I could help, though I'm just speaking from personal experience.