These steels do not like depleted oxygen environments. Its why you see pits in shafts inside cutlass bearings that are left stationary for long periods in water for example and we had lots of trouble in oil tanker cargo tanks with pitted ss valve rods when inert gas at 3% oxygen or less was introduced in the 1970s to stop us blowing up.
A2 and A4 are merely the german specifications equivalent to the uk 304 and 316
both depend on oxygen to maintain the protective coat that makes them stainless. without oxygen, they corrode at least as badly as mild steel. since it is unlikely that you can put an epoxy coat on the metal that will give 100% coverage and remain like that, then leave stainless bare.
incidentally, the problem with oxygen means that you should never use stainless in wood that might become wet, or indeed in grp below the water surface