How can Doru Doru work against seastone We have twice seen Mr.3 open seastone cage/cuffs with his wax powers, once in Alabasta and once in the war of the summit. How can his wax key not melt once inside the keyhole?
As stated in seastome wikia


  Seastone only works upon direct and constant contact.


Which means it works only on devil fruit user not on the power or material they produce using power.


  It has been shown that although a Devil Fruit user loses his/her powers when in contact with the Seastone, said powers can still be used on and affect the Seastone. Example is when Mr. 3's candle wax key could open Seastone cuffs and cell doors, though this may be because the locks themselves are not made of Seastone or because the wax has no Devil Fruit powers inside it, as shown when he unlocked Ace's Seastone handcuffs. Another example is when Shiki levitated battleships in Marineford, despite the hulls being lined with Seastone. Trafalgar Law was also able to switch Seastone chains with regular ones, using his abilities.


Another example is Aokiji Kuzan's ice formation power, which he uses it to make ice road on the sea to travel in his bicycle. Here the ice doesn't melt just because it came in contact with the sea.

And this is just the guess but seastone might melt if comes in contact with Akainu's lava where the temperature of lava is greater than melting point of seastone.