Mastic tape sealant, the original marine grade Freeman deck fitting sealant.
Deck fittings should have a sealant between the fitting and the boat to prevent water seeping into the superstructure and causing potentially serious structural problems and inconvenient aesthetic issues - mastic tape generally serves this purpose with exceptional results.
Boats can take a lot of abuse, bobbing around while moored up, little knocks and bumps and perhaps great stresses while being lifted out of the water. Many sealants cannot cope with such movements but mastic tape, due to its material properties, is able to adjust to these movements. Its pliable nature means it is able to stretch, contract and move with the boat, not against it - maintaining a complete seal.
As a testament to its exceptional properties Mr Freeman used this mastic tape on the very first Freeman Cruiser and the very last!
It is extremely important to maintain the seal on all deck fittings and reseal anywhere the membrane may have perished as this will help to prevent further serious problems - it is worth doing!