..if you can’t get lead compounds
A New Zealand customer offers an alternative to lead-based stopping/caulking compounds for those of you no longer able to get hold of or use traditional materials. He says “Older caulked seams will never allow any poly …type sealant to adhere properly ’cause the edging is saturated with (linseed) oil in which the red lead or white lead was suspended. So use ordinary glazing putty in linseed oil base, mix judiciously with oil based antifouling primer, fill caulking gun and fill seams. Then paint over with antifouling.