How hard or easy are Zipp rims to build up? I have a set of Zipp 303's, but I want to rebuild the rear wheel with a new hub.

I've built many alloy wheelsets before, the last was with Kinlin Xr-300's, but I've never built a carbon wheel. I read the building tips on the Zipp website and they mention lubricating the nipple shoulders, what type of lubricant should be used?

Also, I have hear that Zipp rims are made specifically for front and rear so you can't build up a rear rim as a front wheel and vice versa, true or not?