Just like Walter, we have comfrey growing outside the workshop and the last time I suffered a grazed finger I recorded the process of preparing the plaster. Moving clockwise from the typical bell-shaped flowers, above, we come to the all important roots which have a black covering. Peel this with a knife then pulverize the root tissue which is applied directly to the cleaned wound. After a while it sets like plaster, and sticks well, so you can carry on working and there are no irksome bandages or tapes to get in the way. The effective ingredient is allantoin which promotes the growth of skin tissue.
Historically comfrey root has been used to treat broken limbs, because it makes a plaster cast, and even recently it has been used for major wounds such as a gash inflicted by a chainsaw.