Usually on the surface all the time without a case of paint forms a film or crust when you remove which are often particles: once on the brush, they can ruin all the work.
Take the time to look for an opportunity to drain the contents of the can. Easier to cover it with a piece of rare fabric, such as old nylon stocking and a little indentation its the same brush: the paint will show through an improvised sieve and be pure as strained.