After washing my paint brushes with soap and water and rinsing them, in order to avoid damage to the bristles and the ferrules and the wooden handle, is it best to hang them bristles down and let water drip out through gravity, or will evaporation cause the water to travel up into the ferrule/handle/bristle and cause damage?
In my experience, in brushes the strongest force acting on the water is the capillary force due to the surface tension in the liquid and the proximity of the hairs in the brush.
Surface tension will cause the water to try to "wet" as much of the brush hair as possible - regardless of orientation. If there is excess water, such water will be pulled down by gravity and could indeed pool inside the ferrule which is not desirable. More details can be found at this website on brush care - for example this advice:
- Air dry your brushes by laying them flat. Non-resilient or especially long tufted brushes, such as squirrel mops or Japanese hakes, will dry more quickly if hung from the handle, tuft down, to encourage moisture to flow away from the handle toward the ends of the hairs.
There is another interesting phenomenon with brush hair - something you have no doubt observed. Hair expands when it is wet - this is true for natural hair as well as for most polymer fibers. As the brush dries by evaporation, the outside of the outer hairs will dry first - and in drying, it will shrink. As a result, the brush tends to get "bed head" -
From the website quoted above:
The best method I've found for drying is to set the handle of the brush in a brush holder with the tuft resting on a clean surface (right). Balance the brush so that the hairs rest lightly and hold their shape. I also rest my brushes in this position when painting, as it drains moisture away from the ferrule and keeps the tips moist for as long as possible. Resting flats in this way helps them dry to a clean, straight edge.
Slow drying while maintaining the shape of the brush head, and preventing the pooling of water in the ferrule, appear to be the key to a long brush life.
You should let it hang downwards, otherwise the water will drip down onto the ferrule, which can eventually corrode it. There's pretty much no reason to leave them tip-up: in particular any "local pocket" of humid air is unlikely to be rising much rather than expanding outwards in all directions.
If you are worried about the local pocket of humid air during evaporation, you can virtually eliminate this pocket by pointing a fan at the brushes, or putting them somewhere that's draftier like an open window, or drying them above a radiator or other heat source (hot air, rising, will form a natural air current). Transport physics in air tends to be dominated by convective processes.