abua said (in comments on pixiv): 貴重なもっふりから、一月は生臭さが抜けませんでした。なんか意味不明なネタでスミマセン；
There's neither mention nor implication of waterproofing or oil in there, nananonina.
It's an obscure joke, one that I don't entirely get, but here's my take on it:
translation: Their fluffiness is precious, so she couldn't get the fishy stink out after the New Year.
At a wild guess, I'd say it has to do with things not drying well in the cold weather of January, thus Ran didn't wash her tails - thus they stink of blood (and thus smell like fish).
Also, for things like this (when the artist's commentary gets posted and/or translated in the comments on Danbooru), use the commentary tag.
