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It depends. When used with latex paint, roller covers can be easily cleaned and reused. First, use the curved part of a 5-in-1 tool to scrape out as much paint from the roller as possible. Then, use the same tool to wash out the paint using a sink or bucket until the water runs clean. Put the roller cover back on the roller handle and spin as much moisture out as possible. Then put it back in its plastic sleeve. If used with oil-base paint, go ahead and discard it.
When using latex paint, you can wrap the used roller in saran wrap and put it in the fridge over night. When you get back to painting the next day, you can take it out and use it as is. (As long as it is the same color that you were painting with from the day before)