Before writing this post, I did a google search about using makeup brushes for painting and almost all the results were actually about using painting brushes for makeup (some even claim it is cheaper). So let me tell you my story. After attempting a couple of half and full sheet paintings recently, I discovered that my 1 1/2 inch taklon (good-ish synthetic) brush was no longer coping when I wanted to cover a large area in paint and achieve a smooth wash. Nor was my #36 round brush (also synthetic). It logically followed that I need a big brush with good paint-holding capacity (i.e. natural hair). If you ever looked for one, you know that even squirrel brushes start somewhere at $40 for 1" flat.
And so I had an idea. I was looking for a powder brush at Target when I saw "natural bristles" on the package. $5, worth a try? If it didn't work for watercolor, I still needed a powder brush, right? But it worked! It holds a bucket of water and paint and it is perfect for covering a large area. It even sort of holds a point when it's wet. So here is the brush (Studio Tools Powder Brush):
And a very quick wash on a half sheet (22x15"). I haven't even tried to make it smooth, hence some streaks. It is still a hundred times better than something I would get with my synthetic flat.
It took me maybe 5 loads total to cover the whole sheet - and not because the brush ran out of paint, but because I wanted to change colors or get more saturation. I was also afraid that the brush would shed profusely - but it didn't. It shed two hairs, and I can deal with that :)