George @ Uvonics:

When a Tin bath goes muddy, whatever color of mud, and it can go from White
to brown, it is usually Stannic Tin oxides, which are the result of (1.)
atmospheric oxidation of the Stannous Tin, and/or (2.) oxidation caused by
too small an anode area.

The Stannic oxides will co-deposit with the Tin metal, and cause the deposit
to be less bright, and degrade the quality of the deposit.  Some of the
plating brighteners cause the Stannic oxides to precipitate so finely that
they are VERY difficult to filter, (do I have to tell you that these are the
ones that co-deposit the worst??) and settling and decanting may be the only
way to get rid of them.   Suggest you try filtering, if already filtering,
try finer filter.  Consult with vendor, if you are not filtering currently,
to determine if this will screw up brighteners. 

When plating with Fluoborate bath, this does not occur, as Fluoborates are
not completely stable, and decompose slightly to Fluoride and borate, and the
small amount of Fluoride present complexes with the Stannic oxides to form a
soluble Stannic Fluoride complex, which is inert, and does not plate out.

Rudy Sedlak
RD Chemical Company
RD Chemical supplies answers, as well as solutions.