Straining eliminates lumps in the paint. If the paint has separated, stir the thick paint up from the bottom of each can to free as many lumps as possible. Then box the paint, pouring it all together through a nylon paint strainer and into the bucket. Paint less than one year old usually doesn't require straining. Older paint might have a thick skin on the top; remove the skin and set it aside. Box the paint, pouring it through a nylon paint strainer into the bucket.