Hmm. This one could be a bit scary, as a couple of posters have already mentioned.
On one hand, I have had some older (e.g. steel 72s) cylinders that I have VIP'd which came in 'empty' - i.e. they were at ambient pressure - and then bubbled around the cylinder neck after filling (and cylinder neck O ring replacement, which we automatically do with a VIP). In those cases, a thorough cleaning of the valve gland addressed the problem. Cylinder neck threads were (by appearance) in good condition, the valve tightened appropriately when installed, there was just a trickle of bubbles under pressure.
But, while I guess I can see how some would consider it practical to apply a 'putty' to stop the leaks, I would not do it on my own cylinders, nor would I recommend it to anyone else. (Frankly, for whatever reason, the notion of applying a grease or putty brings to mind some testimony I vividly recall being presented by Lawrence Mulloy during the Challenger disaster inquiry). If the cylinder is leaking around the neck, I agree with halocline and others - address the problem (O ring, corrosion, neck thread issues, whatever), or don't use the cylinder.
The scary part is that you could have a damaged valve, or cylinder cracks.
