Plastic for water, natural for oil

One of the little rules that my Dad taught me was to use synthetic bristle brushes for water-based finishes and natural bristles for oil-based finishes. AFAI can recall his reasoning had to do with the nature of the finish and whether the fibre took the finish up or not. Basically, natural bristle will take an oil-based finish quite a way up the brush, even if you only dip it a little way into the finish, and this allows it to flow on, but that with water-based finish, which doesn't flow out so easily, you get a better finish with synthetic because you can get more of the finish on your brush onto the work-piece because it doesn't soak into the bristles. It also makes cleaning easier for the same reason.

I thought this was just the old man crapping on, until I heard Hendrik Varju on finishing on a couple of Matt Vanderlist's podcasts (this and that) and he said the same thing.

So then I tried it and I reckon the old man and Hendrik are right.

The next thing was how to tell which brushes are which. For that, I decided to use the synthetic brushes that have coloured bristles - usually blue - and reserve traditional back for natural bristles. Simple.

Your dad (and Hendrik Varju) are quite correct. It's been a very long time since I painted anything, but I'm pretty sure most oil-paint tin instructions advise the use of natural bristle brushes for the reasons you state.

I always use synthetic filament brushes for water-based stains and chemicals because the acids and alkalies don't eat them up.