Homebrew PCBs

Hi everyone.

A newbie to this group, I am (perhaps over-ambitiously) attempting 
to set up a micro through-hole plating line at home.  It's a lot 
more involved than I ever thought it would be, but I now have most 
of it working fine.  ...Except what should be the easy bit - the 
final stage - the etching!


To give a brief description of what I do...

STAGE 1:  Starting with plain, copper-clad board, I do the drilling.
STAGE 2:  Apply photopolymer laminate, then expose & develop. (This 
is positive photopolymer, so the copper which will ultimately become 
tracks is exposed to the air on developing the pattern)
STAGE 3:  Go through a 6-stage process to electroless-plate the 
entire thing (including the holes & edges). The copper cover it puts 
on is pretty thin, but uniform at 1-2 microns.
STAGE 4:  Electroplate the copper - this gives reasonable thickness 
to the copper applied in stage 3 - I aim for 25 microns.
STAGE 5:  Electroplate with tin to 10 microns.  This 
(theoretically!) protects the copper from etchant.
STAGE 6:  Remove remaining resist.  Then etch.  This should leave 
the tin-plated areas untouched, everything else should be zapped.


But alas no!  Stage 6 fails, because both tin *and* copper are 
etched, leaving me with a fine, blank piece of fibreglass!  It 
matters not whether I use Ferric Chloride or Sulphuric/Peroxide 
etchant - they both destroy what has been so lovingly created!!  So 
I reckon there must be something awry with my tin plating.

It's a total impasse, and nothing I do, from increasing/decreasing 
current to re-formulating the tin plating bath according to 
manufacturer's instructions seems to change things.

Does anyone else use a similar process, or have any clue about what 
might be the problem here?

Max.