Naughty Chair - What to do?

[Calluna]

I would have said the same as ss-storm, but if thats not working you could go back after the initial time and explain that the undressing is not ok and because she did it she has to stay for another 30 seconds or whatever you think is reasonable.

Eta- probably better to have your conversation about the original issue then tackle the undressing otherwise she might get a bit confused.
So she misbehaves
goes to time out
undresses herself
time out finished
talk about behaviour
then talk about undressing
and do a second time out.
Hope that makes sense