If you make molded candles, whether in rigid pillar molds or in flexible molds, you will have to find a way to prevent the melted wax from escaping through the wick hole and turning your work area into a big wax puddle.
To help you achieve that goal, your best ally is the mold sealer, aka candle mold sealer putty.
Metal molds sometimes come with their own sealing device, either a screw or a rubber plug. As far as I'm concerned, I don't like either option: the screw tends to damage the wick and with my molds, rubber plugs are a pain to insert and usually don't stay where they're supposed to, which has led to a few accidents...
Mold sealer is a putty-like substance, usually white, slightly sticky and completely waterproof. You can find it in stores that specialize in candlemaking supplies but that is not the place I suggest you buy it from: it is generally overpriced considering the alternative.
Instead, go to your nearest do-it-yourself superstore and buy a small pot of plumbers putty. Plumbers putty does not harden and keeps its elasticity, which is exactly what we want. A 14 ounce pot will set you back about 8 dollars and will last you for years.
Do not hesitate to use it in abundance when needed, it will save you a good deal of grief.
Whenever you use two-pieces molds, like those designed to create a sphere candle, I highly recommend you cover the whole joint with a generous amount of mold sealer. Failure to do so may result in a half-sphere candle and a messed up countertop.
Another advantage of mold sealer: it will prevent water from entering the mold and mixing with the wax when you need to dip the mold and its contents in a water bath.