Rusting is the other thing I was thinking, but wouldn't the wax prevent evaporation and leaking?
Sure, but to what end? What's the goal? To keep liquid inside? I mean, if the liquid inside the cap/cork wants to get outside, then waxing is just going to end up with a metal cap sitting in an acidic solution for a long period of time, which seems obviously terrible to me (the cap will corrode and now has an easy diffusion path back into the bottle). Is that worth keeping some small amount of liquid in the bottle? I dunno, I don't see why it would be.
As
tehzachatak said wax is gas-permeable, so wax won't prevent oxygen infiltration into the bottle, though it may slow it down. But, again, does it matter? Lambics seem to be pretty robust to oxidation, and elemental oxygen actually isn't a big deal for oxidation anyway. I've read that it requires metallic ions to form into a free radical that can actual oxidize something, so to take it back to my point above having a rusty cap sitting in solution would be bad for a second reason: the iron atoms can now convert the elemental oxygen to a free radical (note: I'm not a chemist and I may be misunderstanding this).
Further along the "to what end?" question, we have the fact that, aside from catastrophic amounts, I haven't heard of leakage harming the beer (I actually just recently had a 2012 3F Geuze that was covered in a sticky film that smelled like vinegar, given that it had been sitting untouched in my cellar for months this pretty much had to have come from the bottle, it was perfectly fine). And if the bottle for whatever reason wants to disgorge half its contents, will some flimsy wax actually stop it? I have no idea, and I'm sure it depends a lot on how you wax it, but in general I'm pretty skeptical.
Finally, I can't see why you'd care at all about evaporation. In fact you'd probably rather that any leaked liquid evaporate quickly into a (relatively) harmless film than sit around and cause corrosion.
So I think that, without actually trying it in some kind of weird experiment, you'd probably rather not wax your lambics. (I can think of one exception to this and that's if you're about to ship something and are particularly worried about the integrity of its seal.
SeaWatchman did this with an old bottle he shipped me that only had a wine cork to keep the contents in, but there the wax served mostly as a mechanical means of keeping the cork in, a large amount of tape would've worked too.)
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