this is one of specific characteristics of lime paint.
You can seal it with "breathable sealer" though - you can get it from adicolor.com
or make your own because it is virtually a casein mixed with a little bit of lime, resulting in casein glue (lime is one of the chemicals that turns casein into glue).
This is how the historic distemper was made
andrea mantegna used this technique (distemper) a lot
learn fresco at http://FrescoSchool.org
fresco painting video tutorials: http://www.FrescoSchool.com
Fresco School Video Channel
Contemporary Fresco Gazette
commission fresco or mural http://iLAdesigns.com
Contemporary Fresco Painting Resouce Center