Hi. The Highway One guitars are finished in satin nitrocellulose laquer. However, you can use pretty much anything to finish a guitar, depending on what gear you have and what you are aiming to end up with. There are water-based finishes that work quite well available from places like stewmac.com and lmii.com, while nitro is available here and there (reranch.com in the States, but they can't ship it overseas). The good thing about water-based stuff is that it can be shipped anywhere and it's less toxic to use. It also dries faster than nearly anything else. The bad news is that the coats don't melt into each other as you spray, like solvent-based finishes do, so if you make a mistake you have to sand it out right then and not rely on being able to complete your coats and then sanding it. So you risk sand-throughs and other cr@p more often.
Duplicolor auto touch-up paints are another decent alternative for finishing guitars. Lots of colours, too.
Hmm. Back to your question. :) Err, in order to get a thin coat, like on the Highway One Strats, I wouldn't thin the paint excessively. You can do it. Some people will do it when spraying non-water-based polyurethane and nitro. But I would just not put many coats on and spray lightly to start with. If you have spray gear then this is pretty easy to accomplish, since you have more control than you do with spray cans. If you're talking about a transluscent finish, then these can be done by tinting clear with your colour of choice. Spraying a finish like this can be a pain though, so you'd wanna practice first, because transluscent finishes (and metallic ones) can often end up patchy where you might have accidentally put more colour down in one area than in another.
Depending on the wood--whether it's light or dark--and the colour you want--whether it's light or dark--you can get good results by rubbing a stain into the wood and then spraying clear over it. You're less likely to get a patchy finish that way, though it does have challenges of its own.
If any specific question occur to you, just ask.
