Has anyone done a significant amount of repair work to the tubes of one?

I recently picked up a 15' Novurania 550 and it has a slow leak. The old owner said it was likely a valve. After looking at the tube bottoms, they could use some work... I'm not convinced it just leaks from the valves...

Specifically, I am thinking of picking up something called Tuff-Coat....Supposedly there is a base coat and a finish coat that is like a liquidized rubber and also a paint. Its $300-$400 bucks but thats better than $4k to get new tubes...

Has anyone ever used it that can give some insight? Also, anyone know where to get the valves and how hard are they to replace?

Thanks.

These guys may be able to answer your question it is a UK based site as RIB's are very common in the UK their is also a USA section
http://www.rib.net/