Black cobbler's wax is handy stuff - wooden-boatbuilders use it (builders of Norse Faerings call it 'boat soup' - heated to near-bubbling and painted onto the wood to soak into and fill the grain and preserve the wood.) as do pipers (Players of the Great Highland Bagpipe only use it to wax the cords that they use to tie-in a traditional leather bag, but makers and players of the Scottish and Northumbrian Smallpipes and the Uileann pipes use it to seal their bellows and bags)
I had been rooting around my shop, looking for that last bit of black cobbler's wax that I was SURE that I had somwhere (not wanting to order some in from the bagpipe supply shop (only available in expensive tiny wee blobs) when I remembered the old recipe.
You'll want to do this outside unless you're lucky enough to be living with someone whose idea of heaven is living in a house that smells like a square-rigged ship!
Equal volumes of bitumen or pitch - if you can't find that, use Stockholm (Pine) Tar - and bee's-wax.
Rosin (available by the bag at any dance shop, or by the cake at any music store that caters to fiddlers) or
Beef suet
Melt the bee's-wax in a double-boiler (to lessen the risk of fire) that you've selected for the purpose because it'll never come clean again - use only enough heat to melt the wax - DON'T boil it!
When it's thoroughly melted, slowly stir in the chunks of pitch or dollops of pine tar.
When the pitch and wax is thoroughly blended, add a bit (roughly 10% of the total already in the pot) of either rosin or beef suet and then stir until it's all blended.
Pour into cakes (I use paper shells and an old cupcake tray that I don't think the wifie has missed yet) and allow to cool.
Rosin will make it drastically more sticky - your thread will grip tight against the leather and other strands of thread - and make it more hard when it cools, and the suet will make it 'greasier' and thus more inclined to soak in.
It took me a while to experiment and find the optimum amount of rosin to add for the task at hand.