Excellent, but take a tip from one who's suffered this: It's best to start with a fine sandpaper, building up to the coarse, green, variety. That way, the pain builds slowly but exponentially, giving your Mistress more pleasure and you more agony.
Almost there... now, soak a paper towel in lemon juice, wrap it around his cock and cover with a condom to keep in place and tight against that roughed up skin. Sit back and watch the fireworks. I recommend tying him down - my Mistress made that mistake the first time she did this to me.