I’ve written about this before in other shoe care articles, that it’s good to let wet shoes dry lying on the side to speed up the drying process and allow the entire shoe to be aired. Recently I became aware of what can happen if you don’t do this, when one of my bespoke pairs from Janne Melkersson was used in rain and were left standing as usual, and ended up with a proper formation of mildew on the sole.
It was after getting caught in a real autumn shower a month ago or so, and without thinking about it I left the Melkersson shoes standing as usual on my shoe rack in the closet. The sole was still very moist, and when the flat portion that is flat against the ground was left so flush with the wood of the shelf without being aired mold was formed. It wasn’t until a few weeks later that I discovered it when I was supposed to use the shoes again.
There were no major problems to scrape almost all the mold off with an ordinary kitchen knife and then let the shoes dry as they should have been doing from the start, lying down, and after a couple of weeks almost everything was gone and with some wear the last got off and the soles were restored. But it’s obviously not ideal to have this happening. So there is every reason to really keep the rule to let wet shoes, not least when they have leather soles, to dry lying down on the side.