Since I couldn't find diddly squat about DDV details anywhere on the net.....
I am starting this one.
I had an old BRAND new bottle in my stash....
That was all firm (dried out).
Anyone who works with paints knows pigments just need to be re-melted back to useable format with (typically) a thinner.
I searched for what others might have used, as the label stated just a generic word "thinner" or "mineral spirits".
Well, I didn't want oils in mine (that I might be adding)....
So I opted for some good ol' Laqcuer thinner.
Took about a week.
But its back!
So theres a confirmed use of this method to bring back an old bottle of DDV(Decal dulling varnish)
I also found a local train shop that had 4 old bottles of this stuff last week.
They all were just as dry!
Hope he doesnt toss em!
Doc