Sentence:  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 <WSD>box</WSD> 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. <WSD>Box</WSD> the paint, pouring it through a nylon paint strainer into the bucket. 
Meaning of the word marked with <WSD> tags:  To mix two containers of paint of similar colour to ensure that the color is identical.