Viakal. Or vinegar. The best method I have come across is to take a small plastic cup of vinegar, immerse the tap in the vinegar and wrap a tea towel around both cup and tap to hold it in place. For limescale build-up around the posts and other parts of a tap, soak a pad of cotton wool in your descaling liquid and wrap this firmly around the relevant parts. Leave it there for an hour or two, giving it a squeeze now and again to make sure the acid gets into all the corners and grooves. After this time, all parts of your taps should be able to be wiped clean, though you may need to scrub with a plastic scourer to loosen the more stubborn bits of scale.