AHas anyone ever tried to paint a buttercream coated cake? I'm thinking of trying it, but I first wanted to ask in case you may have tips.

Don't paint it immediately, allow it to crust(harden) before u start painting it.

Yes, allow the buttercream to crust and paint with luster dust and vodka. Do several light layers for dark colors rather than one heavy coat as it will moisten the buttercream.

http://www.cakecentral.com/g/i/3249423/a/3490769/extended-tier-cake/