Here is an example of what happens when the varnish eats your image (above). In this case I had just added (too much) Everclear (190 proof alcohol) to the varnish to try and solve a different problem – and the collodion on this plate is pretty old – so when I poured the varnish on the image just started to flow off the edge.
Here is the pre-varnish scan:
This is also why you do not start a varnishing session with your best image from a shoot. That dark spot on the right is because of something I did wrong during the session, not bad collodion or bad varnish or something like that. So I save that plate for varnish testing – if something like this goes wrong you at least have a chance to do some troubleshooting for the remaining tintypes. (The ruined one is not the good one I have for sale.)