The last tank I cleaned I ended up cutting access holes into the top of it to get the varnish out. What I found that worked well was seafoam, I wet a rag and coated the inside walls. Once it sat and soaked in after a few minutes it loosened up the varnish where it wiped right off. You may be able to fill the tank with a bottle or two and soak it then shake to get most of it out but I have found that the only way to fully clean one is to get you hand in it to wipe all areas.
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