The block and exhaust manifold of the A4 are cast iron.
When continually and repetitively heated above 150°F the internal 'rust' will become a blue/black form of rust (ferrous oxide) which is 'protective'. With long periods of lay-up, this ferrous rust will convert back to destructive red (ferric) rust.
Rx to keep the rust 'black':
1. run hell of the engine, keep the water temp at a bit over 150° (regularly check that raw water thermostat for proper operating temperature).
2. Never, never, never, ever drain the water from the engine for long term 'lay-up'; always fill up the cooling system with 50:50 antifreeze mixture with added 'anti-rust' compounds. The oxygen laden moisture remaining in a 'drained' cast iron engine will greatly
accelerate the 'black' rust rapidly converting into destructive 'red' rust.
3. Never, never, never, ever 'pickle' such an engine or exhaust manifold with muriatic or hydrochloric acid (or 'vinegar', etc.) to remove the usual
internal calcium carbonate scale buildup/fouling. ONLY 'descale' the engine with an INHIBITED
descaling chemical such as RydLyme
, etc. Then run the hell out of it to restore that protective ferrous black rust.
4. Convert the engine to 'fresh water cooling' (heat exchanger).
Good source of info for maintenance, repair, rebuilding, parts, 'modernizing
', etc.: Moyer Marine Atomic 4 Engine Rebuilding and Parts