You can get valve lapping compound in any automotive store - it's used for grinding the valve seats of cars, motorbikes, tractors, marine engines etc. Get a fine paste.

Just to be clear, I'm recommending this for the steam wand valve, not the safety valve. I certainly wouldn't use it on the new nylon head safety valves.

Apply only a small amount to the tapered tip of the valve stem and rotate in one direction then the other, then clean throughly, using a small cleaning bud (preferably not the cotton wool type which can unravel). Once you have greased and reassembled the valve using food-grade grease (Dow grease available from Orphan Espresso), give the wand a good flush through with water (machine off!) and then subsequently with steam (machine on!).

Before using grinding paste, try just giving the valve a good clean (the leak problem may simply have been caused by some scale or other debris, and rotating the stem in the bore (but without any grinding paste). Then clean again. This may be enough to have the surfaces mate up more effectively.

Cheers

Mike

[Edited to provide more detailed advice]

The valve shaft is chrome. Will the lapping compound ruin the finish?

Tony

compound is just for the seat ( where the valve closes) you will not be using on the shaft.

Thanks for the suggestions. In the end, I fixed the problem by using some fine steel wool on the end of a small brush, dipped in a descaler, which I rubbed along the valve seat. I got the idea from Orphan Espresso (they suggested steel wool and citric acid). No more leak. Now if I could only solve the weak steam problem (hmmm.)...