Tracy,

While at Naval Avionics we experimented extensively
with electrolytic tin plating in late 1980's.  We tried
numerous ways of minimizing undercut including
starting with 1/4 oz foil and selectively plating the 
tin areas then etching with no protection to the other
copper conductors.  You only have to remove the 1/4
oz copper so etching is very fast.  Of course this
works on paper and on prototypes, but when you
get to actual production with the variations in
plating thicknesses and all the other subtle miriad
variations - you get slivers.

I spent a lot of time manually brushing boards to remove
the slivers just to salvage boards for engineering 
development.

Sorry my response isn't more positive than the last.

Susan Mansilla
Robisan Laboratory
317-353-6249