Can I say no if I am a cashier/clerk asked to paint walls?

I just started a second job as a cashier/clerk a a chain grocery store in Chicago three weeks ago. Yesterday we had a store meeting and were told that the employees had to paint the whole store from top to bottom. When I accepted the job I was told that my duties were to be cashiering ,stocking shelf's,and getting carts. I looked in the employee hand book and it says nothing about the job description just the rules,and regulations.(ual Harment Workman's Comp things like that) I don't have a contract it is employment at will. Let me just say I don't mind doing it but do I have to?