But you don't want to use new pennies since the zinc will show through the design. I've also heard that the copper starts flaking off after awhile too.
I have cleaned pennies myself now. What I did was put one in a bowl of vinegar with a little salt in it. Once the grime came off, I would wipe it off and then use the baking soda/water paste and buff the penny to a brilliant shine. I'm glad I didn't wait till last minute to do this since it does take some time.
You don't want to put more pennies in the bowl then you can clean in a 3-5 minute time frame. The grime will transfer if pennies touch. Also if you put in one that has the zinc in it, you will know. My hubby brought home a 1982 one and I figured I would try to see if this was a zinc one or not. Yeah, it turned pitch black. I ended up throwing it away because it smelled horrible.