Lo siento!
Yes, I have been down the leaking tap seal road. Nowadays I stick with barrels that have the tap at the top and a float system to drive the fluid up under pressure. I was a bit apprehensive at first - the more complex the mechanism, the more there is to go wrong. But so far (touch wood) they have worked very well for me and cut out the liquid loss through the tap.
I was advised by the guy in the shop that a smear of plumber's cement helps. Tried that, and it didn't. Didn't really expect it to. If the leakage is being forced out under a head of pressure then short of sealing the leak properly, nothing would stop the seepage other than reducing the head of pressure, and plumber's cement is never going to do that.
I did find that tightening up the tap screw beyond the recommended finger-tight did actually stop it in my case on the one barrel that I still use with a bottom-fitted tap, but I do worry that the whole thing will explode one day when the thread finally gives in.
What I would really like is some nice brass taps, and not have to rely on this cheap plastic. I would be prepared to pay a premium for them, but they just don't seem to be available.
Here is another quick tip, though: If buying a new plastic barrel, unscrew the tap from it before you commit to buying it, and have a careful look at the positioning of the hole that has been drilled into the reserved flat circular area on the barrel. If they didn't do it carefully you may find that the hole has not been drilled central in the area, which can have the effect of not leaving sufficient plastic all the way around the hole in order to bind properly with the rubber washer seal attached to the tap fitting.
In my problem case it is a new barrel, so should not require a new o-ring just yet. Hence the planned trip back to the supplier