[alancalverd]

I polish my shoes with Kiwi Parade Gloss (other premium finishes are available).

Standard procedure: remove loose dirt, wipe with damp cloth, leave to dry, apply polish and brush to an even shine.

Now comes the bit I don't understand. Sprinkle some water on the now water-repellent waxy surface, and brush again. This produces an almost mirror-like gloss, as every soldier knows.

What's going on in the last "spit and polish" phase? 

[Petrochemicals]

Probably the thin film hardening once exposed to the atmosphere, in a similar way that old candles become strange, thus allowing a better shine. It could be bacteria, it could be oxygen or something else.

[alancalverd]

But why does it only happen with a bit of water brushed over a hydrophobic surface? The effect is immediate.

[Eternal Student]

Hi.

I don't know...
    A google search brought up this statement from one shoe manufacturer:

The layer of water between the wax layer on the leather and the shoe wax on the polishing cloth reduces friction and smoothes the wax layer. The result is a higher gloss.

   Seems to suggest the main reason is quite mechanical rather than any elaborate chemistry going on.  O.K.  you need the polish / wax  to be hydrophobic and that I suppose is a chemistry thing - but then you just get the benefit of a good slip.

   Without the water, I guess some small parts of the polishing cloth are stalling (stopping or biting in), grabbing a bit of polish and moving it somewhere.  You have the usual sort of thing where   coefficient of static friction > coefficient of dynamic friction  as usual.   This sort of "biting in" is presumably happening on a very small scale but often and the result is a very un-even coat of polish or wax is left on the shoe.

Best Wishes.

[alancalverd]

Well that makes sense. Thank you!