It is not the pin that needed to be sanded. But the outer diameter of the valve just in front of the o-ring.
But if you screw it all the way and it leaks more, then your problem is diferent from the one I mentioned. Donīt sand. Sanding would only help to allow the tank to screw in all the way.
Does the tank screw in very esay? Does it seen loose? It should offer some resistance. I had a similar problem with another tank. I solved it changing the o-ring. You could try an o-ring with a larger outer diameter.
Ambush advice is good too. If you can find another tank that match your SP1 try to trade.
Iīd try your tank with another SP1 also just in case. Your SP1 ASA adapter may be wider than normal.