Coating

Coating is the process in which a ship is coated with special resin from the Yarukiman Mangroves, enabling it to survive the crushing depths at the bottom of the Red Line and other extreme depths.

Overview
Coating is performed at the Sabaody Archipelago, where the Yarukiman Mangroves reside. It is done to allow ships to bypass the Red Line, which is otherwise impossible to cross, through Fishman Island. Not even the most astute submarines are capable of such a dive, since Fishman Island rests on the sea floor. There is presumably a similar place to coat ships in the New World, given the passage of pirate vessels to and from that half of the Grand Line.

Coating surrounds a ship in a bubble-like membrane which renders the ship less buoyant than the water around it, causing it to sink. Coating is a very delicate procedure and must be done carefully, as any error in its application could compromise the bubble during the trip, causing the ship to be crushed by the pressure if it is deep enough.

Coating Mechanics
A coating mechanic is a person who coats a ship with the special resin that allows it to submerge. It is an expensive job and can take several days to complete. It is unknown how coating is done on a ship, though Silvers Rayleigh has been shown carrying several tools presumably related to its application, including an oversized brush.

Using a coated ship
Once a ship is coated, it loses the ability to stay afloat. Until the ship is read to depart, an air-filled 'floating bag' is used to keep it on the surface. Once the ship is ready to leave, a valve connecting the bag to the resin bubble is opened, transferring the air into the bubble, causing it to cover the ship completely in order to provide breathable air for the crew. The completed 'bubble roof' also serves to protect the entire ship from the extreme water pressure beneath the sea though it seems that, just like the bubbles of the Saboady Archipelago, it can be penetrated without popping by people.

In the anime, the bubble is shown to pop when a ship reaches the surface once more, though this may have been done intentionally by the crew.

Navigating a coated ship
Once the floating bag is removed, the ship starts to sink. However it does not mean it will sink straight down. The ship is to be navigated by using the under water sea currents as it would use the wind on the ocean surface.

Coated Ships

 * Moby Dick and Whitebeard's other four ships
 * Thousand Sunny
 * All ships owned by the Eleven Supernovas