Here is something that I just found on my Eco-Cruze with polished rims. I suddenly was seeing slow leaks on two of my tires after adding air. I pulled one tire off to see if I picked up a nail, and saw nothing. I then sprayed soap around the bead and around the valve stem to the rim. I found that on both of the tires I was leaking around the valve stem. I'm not sure if this will work in the long term or for everyone, but I sprayed WD-40 around the valve stem and wiggled the stem back and forth and twisted it as much as I could without damaging anything. I reapplied the soap, no leak. It looks like there is micro-corrosion around the stem that is opening up a small leak path between it and the wheel. The WD-40 crept back in there and seems to have healed it for now. I'll update if the leak opens back up.