A. Depends on what you're trying to do. For most watercolors, I like kolinsky rounds and sabeline or light oxhair flats because they hold so much water and paint. If I'm working really wet, synthetics seem to give me more control without flooding the paper. There are a lot of brushes available now that are blends of hair and synthetics, but you do have to be aware that some contain more synthetic fibers than hair. For acrylics, I use synthetics and bristle brushes, because I don't want to ruin my good watercolor brushes, which are generally too soft for use with acrylics anyway.
Oil painters can use all three, according to their preferences.
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