If maintained properly they are. First is more like a primer or a paint there is also a compund in joints to prevent leakage.
Generally fuel cells are not coated, primed, or painted internally. The most common finish in a fuel cell is bare metal, although some systems do use chromate or other coatings. The basic corrosion preventative in aluminum sheeting is alclad, or pure aluminum coating. This is often treated with alodyning or other methods of corrosion prevention, which is really a surface treatment of actual corrosion or oxidization. This surface oxidization prevents further penetration of the metal. It's a very common method of treating the base metal, and in many applications is followed by other treatments such as paint, chromate, etc.
Putting surface treatments inside a fuel cell can have dangerous problems if the treatment becomes free in the fuel. Paint flecks can plug a fuel filter as easily as ice. Very specific sealants are used inside fuel cells, but generally the cell isn't coated internally with paints (epoxy, or otherwise). Where a full surface treatment is required in a cell, sometimes a bladder is used in place of sealants, and the entire fuel cell is lined with a material which is resistant to the fuel, as well as conventional corrosion. One should note that in such cases, often the bladder material isn't resistant to additives such as Prist; the proper mixing and use of those additives is crucial to ensure longevity and proper function of fuel system components.