Coating

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

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 both halves of the Grand Line.

A 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 oversized brushes.

Using a coated ship
Once a ship is coated, it loses the ability to stay afloat. Until the ship is ready 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. The bubble is permeable, just like the bubbles of the Saboady Archipelago, and will retain its integrity if objects pass through it, even underwater. This includes people and even cannonballs if necessary. However, if punctured in too many places at once, the bubble will pop.

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. Assuming the bubble pops as a result of surfacing, this would suggest that coating only lasts as long as the ship remains beneath the surface.

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 underwater sea currents as it would use the wind on the ocean surface.

Due to the bubble coating's nature, the ship can fire cannons to defend itself from sea monsters without compromising the barrier. However, the bubble's strength is not absolute. It can be compromised if the ship is bitten by a large animal (such as a Sea King), hitting rocks or a trench that cause the ship or mast to hit the bubble, or if sliced by a sword (such as Zoro did).

According to a sheet of instructions left by Rayleigh to Nami: "70% of all ships that head for Fishman Island sink before reaching their destination, so be careful".

Coated Ships

 * Moby Dick and Whitebeard Pirates's other four ships
 * All of Whitebeard's allies' ships
 * Kid Pirates ship
 * Hawkins Pirates ship
 * Drake Pirates ship
 * On Air Pirates ship
 * Bonney Pirates ship
 * Firetank Pirates ship
 * Fallen Monk Pirates ship
 * Thousand Sunny
 * Unknown pirate ship cut by Roronoa Zoro
 * Caribou Pirates ship