Zinc castings get a really tight oxide coating due to the chemistry of the alloy.
They will be slow to start reacting to acids, particularly reducing acids, but when they do they will dissappear before your eyes in a matter of seconds.
The best thing that I have found for zinc parts is a good boil in dishwasher detergent.
They will get a light coating of hydroxide ( because the detergent is alkaline) which is medium grey and will rub off with a tooth brush dipped in warm water.
To get a brighter than new finish , follow the boil with a soda blast.
The light metals are all highly soluable in oxadizing solutions which in laymans terms are most acids and all alkalis so you have to be careful with aluminium and zinc based parts.
Iron OTOH is almost insoluable in strong alkalis so the best way to clean up your barrels or head is to boil them in caustic soda which will also degrease them as well and if left in long enough strip off the paint.
