Many years ago I can recall a job where we took very thick multilayer boards 
(>.125) and ran them through an immersion tin process. Immediately after 
drying with an air hose, we placed the boards in a vacuum bake process to 
remove moisture from the board in a condition where oxidation could not 
occur. Immersion tin remains solderable for only a very short time. 
 Immediately after that process, the boards were immersed into a product 
made by Alpha metals. I'm not sure of the number but, it was something like 
Alpha 826 flux coat. Anyway, you take the boards out of the flux coat then 
"bump" the excess flux out of the holes and then literally hang up and dry. 
The flux coat dried on the board and then was placed into plastic bags for 
long term storage.

The big problem with using this process today is that it used rosin fluxes 
for flux coat and for the final soldering process. Back then were we able to 
use CFC's (Freon) to remove the flux residues. I'm not sure what kind of 
chemistry is available today with respect to flux coats. All I can say is 
that we were able to solder immersion tin boards quite readily by protecting 
the tin with a flux coat.

Bill Kasprzak
Moog Inc.
(Sorry, no "whizzy" graphics) 

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