Rim Brake Tar
 
Much larger block than supplied elsewhere.
The key is not to apply too much, only a couple of short (20-30mm long) lines equally spaced at 6-10 points around each side of the rim is plenty!
Tar can be used on both smooth rims and ground rims but is not compatible with disk brakes.
 
Approx 50-60mm diameter and approx 20mm thick.