Boottop paint was used in the area from the dll (deep load line) to the lll (light load line) or the belt.
Antifouling paint is used below the water line which consists of cuprous oxide and other biocides to resist barnacles and weed.
At the time there was no Antifouling resistant to U.V., so the belt area was coated with a non poisonous paint.
Today's Antifoulings are all weather and U.V. resistant so boottop paints are no longer used at all. At the same time you also create resistance against weed when vessel is loaded.
In this particular case you can clearly see the slime and/or beginning of growth in the black art which has no Antifouling properties.
ON WARSHIPS IF A SHIP HAS A FULL BLACK FUNNEL TOP IT MEANS THE SHIPS IS COMMANDED BY THE SQUADRON SENIOR CO, IF IT HAS A HALF BLACK FUNNEL IE A THICK BLACK LINE AROUND THE FUNNEL IT IS THE SECOND SENIOR SQADRON CO
But in the modern RFA it means Shaun has been there!
Squadrons no longer exist.
The paint used for boottopping was "Paint Black Pocoptic" which did have some anti-fouling properties.