I would suggest exterior enamel.If you are after transparent look of the paint, then put the pigment directly into the sealer.
If you are looking for the UV protective sealer - do not waste your time, use non-fugitive paints - aka paints made of natural pigments.
learn fresco at http://FrescoSchool.org
fresco painting video tutorials: http://www.FrescoSchool.com
also visit
Fresco School Video Channel
http://youtube.com/FrescoSchool
Contemporary Fresco Gazette
http://trueFresco.Org
commission fresco or mural http://iLAdesigns.com
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