It's been a while since the original post so I'm not sure if this is relevant to the original poster, but this might be informative to others.
We're dealing with the exact same issue. Freshly painted base board on the outside of our house showed strange chew marks. We repainted and the same thing happened. So I setup a webcam:
I bought a bag of habaneros and grated them on it. That stopped them, but not sure how long it will last with rain etc. Went to the do it yourself pest control place to buy some of the squirrel repellant (Ropel), thinking it might work better, but they said it didn't work very well. He suggested I just use the habaneros if it's working.
I would like a longer term solution, and am thinking about mixing pure capsaicin (the chemical that makes peppers hot) into the paint. Wouldn't take much, so I don't think it would effect the paint.