Final answer:
Lead-based paint is not allowed to be applied to scaffold plank due to its toxic properties and health risks to workers. Other safer alternatives like polyurethane, acrylic, and epoxy are permitted.
Step-by-step explanation:
The coating that is not allowed to be applied to scaffold plank is Lead-based paint. Scaffold planks are subject to safety regulations that prohibit the use of lead-based paint due to its toxic properties and the health risk it poses to workers. Other coatings such as polyurethane, acrylic, and epoxy do not present the same health risks as lead-based products and are generally considered to be safer alternatives for treating and protecting scaffold materials.