Mending cracks in a panel
If a painted panel breaks you can glue it well on the reverse, but to fill in the cracks properly in order to paint and repair it, you must not use any glue , which weakens in damp weather and would swell when touched with the oil of the painting. But have some white wax, which is harder than the other sort. The oil mixed into the wax prevents it from melting in the sun. The wax must not be hard, but soft as if recently cooled. Apply it on cracks and crevices with the tip of a knife, then scrape.