A great addition to your watercolor equipment and supplies is a working mat. As you are painting and trying to decide ... is this painting complete? Use a pre-cut mat to lay over your artwork to help you make your decision. I have several of these working mats in a variety of sizes. These are mats you can use and reuse and not worry if you get fingerprints on them or splash of paint. This is not a mat to use when framing a completed piece.
You can cut your own mat. You can also find an inexpensive mat in the reject or bargain bin at your local art supply store. They often discount mats they cut in the wrong size or color for a customer. Keep a working mat on hand when you are painting.