FFXIV Hannish Varnish is a crafting material introduced in Patch 6.2 during Endwalker. It can be acquired in exchange for 20 Allagan Tomestones of Astronomy.
Specifically, players should speak to the Tomestone Exchange NPC in Radz-at-Han: Cihanti (X: 10.8, Y: 10.4). For reference, she’s located in the Western Balshahn Bazaar — to your lefthand side as you enter from the Radz-at-Han Aetheryte Plaza. Speak to her and select the option “Allagan Tomestones of Astronomy (Other)” to see the Hannish Varnish available for purchase.
Two pots of Hannish Varnish are needed alongside five Paldao Logs to craft one piece of Paldao Lumber. This is a Level 90 master recipe that requires the Master Carpenter X tome to first be used on the player intending to craft it. Paldao Logs are then used in a variety of other Level 90 master recipes also tied to X-level tomes. Such as the Rinascita Composite Bow.
Assuming the maximum cap of 2,000 Tomestones in players’ inventory at one time, it’s possible to purchase 100 pots of Hannish Varnish in one trip. Though there are several other types of Patch 6.2 crafting material also needed for Level 90 gear.
There is currently no other way to gather, purchase, or craft Hannish Varnish in FFXIV. Though players can trade it on the in-game Market Board.