If you are looking for a spray on finish, I know a really good product, however it is expensive, and if you only plan to build a few, I would suggest it.
The stuff is from M L Campbell To do the first filler coats, there is "Level"
and to do the finish coats they have this stuff called Krystal.
I have worked with this stuff personally, and it is really great. Sprays on light, and covers well, also it sands off easy for quick surface leveling.
Here is the website:
http://www.mlcampbell.com/pages/home.asp
Like I said though, it cost about $50/ gallon, so if you only plan to make a few ukes, or you don't have a compressor and a spray gun, then this stuff would'nt be the way to go.
For a more low tech method, I would also suggest using Tru oil. It can be gotten at any gun or sportings store.
It can make a very thin, and good quality finish.
Hope this helps.
Aloha
Bob