I had a problem (War of the Ring figures still tacky 2 years after applying Games Workshop matt varnish).
I tried to fix it on my own (by dry-brushing over the top of the varnish - no good).
I asked the BoardGame Geek community about it (here).
They suggested an answer (Testors Dullcote).
I tried their answer (ordered a bottle of Testors from here - applied it to a few Isengard orcs - it works! - they look great too).
I love BoardGameGeeks!