No strain, no gain
Transferring finish from the original can to another container helps in two ways. First, you're not contaminating your original supply with debris from your brush. Later, when working from a partially full can, strain the finish as you transfer it to filter out debris and congealed clumps created as the finish reacts to oxygen in the can. Cheesecloth or a fine-mesh paint strainer available from many paint stores and home centers will catch most contaminants. When you complete your project, discard the unused portion, along with any dust and bristles it picked up during use.