i use foam brushes all the time to apply varnish - they are great because they're so cheap and disposable, but they are also a huge waste. i came upon some gray foam the other day, and i decided to make my own brushes with it! using a wooden clothespin as the handle, it's easy to do and you can reuse the wood indefinitely.
Step 1: cut the foam
you can cut the foam with a knife or scissors