Hang on ......
The older, non removable weir versions come with three gaskets on the connecting bulkheads. One is under the flange inside the skim box, the second goes between the skim box and the inside of the tank wall and the third goes between the outside of the tank wall and the external scupper box. Do not put a gasket under the nut inside the scupper box. I have one of these older versions. With the newer, removable weir versions I understand they come with just the one gasket which should go between the outside wall of the tank and the external scupper box.
From your picture, and assuming you do have a gasket between the tank and the external box, my guess is that the 'textured' paint is preventing a good seal. You may need to clean it off around the hole to be just plain glass to get a seal. Do you have the drain lines supported? Also possible they are torqueing the box slightly causing it to leak.
