I don't think i would change the hardener, its still going to canalize, it will just take it longer due to the cooler weather. If you are using the acetone for thinner I don't think I would change, but you may have to use lighter coats since it will take longer for the acetone to evaporate and the paint to start tacking up. if you are using some other reducer, you might check to see if you can get it in a different "temperature". Some reducers come in different formulas that are for warmer or cooler conditions. Basically the difference is in how fast they evaporate. I don't think acetone comes in different temperatures,, but I could be wrong.