A flexible balsa back has been used to stabilize a panel painting that is exposed to wide and unpredictable temperature and relative humidity changes. A tondo by Botticelli from the Baltimore Museum of Art which had been treated for structural instability previously without success, is used as an example of the treatment. After cleaning the surface of the reverse, a bulking bonding material, a moisture retardant/bonding layer made with PVA (two grades), kaolin, and calcium carbonate, was squeegeed onto the tondo and a piece of open weave fiberglass embedded into it. Balsa wood blocks were arranged on the reverse and adhered with a mixture of Multiwax W-445, Zonarez B-85, kaolin and calcium carbonate. This mixture was more alkaline than the original support.