I don't know if adding a correction layer can be 
called restoration. shouldn't it be done one the 
cel itself ? What if you have a 5-layered cel ? 
Will you get 5 correction layers ?
I believe it's an interesting intermediary 
solution (a lot cheaper as you said); but not 
really restoration.

Those professionals I was talking about can do 
many other things, such as removing acidity in 
the cel to slow down aging or improve the 
surface of the cel to remove scratches for 
example.