What is recommended for coating the inside of an empty bladder type fuel tank for an extended period?

Using engine oil is a recommended practice for coating the inside of an empty bladder-type fuel tank that will remain unused for an extended period. Engine oil provides a protective layer that helps prevent corrosion and oxidation of the metal surfaces inside the tank. Its physical properties, including resistance to water and moisture, contribute to safeguarding the internal components from environmental factors that may lead to deterioration.

In contrast, some alternatives might not offer the same level of protection. For example, water-based paints may not adhere effectively in conditions where moisture is present, which could lead to peeling or flaking over time. Ethylene glycol, while useful in some applications, does not serve as a protective layer and could lead to complications if it contaminates the fuel system later. Linseed oil can provide a protective barrier but is not typically recommended as a long-term solution due to its ability to oxidize and form a sticky residue, which could attract dirt and debris. Therefore, engine oil stands out as the best option for long-term protection of bladder-type fuel tanks.