I have done this several times on my Universal 5411 diesel using oxalic acid. Here's some instructions I found on the Internet for the A4:
There is a simple way to clean all raw water cooled engine blocks. It involves circulating an oxalic acid solution through the block until the scale is dissolved, then neutralizing with baking soda.
1. Close the raw water intake valve.
2. Remove the water
pump impeller and put back the cover.
3. Disconnect the water intake hose from the intake valve.
4. Remove the themostat. Replace the cover.
5. Disconnect the water hose which injects water into the
exhaust.
6. Fill a five gallon bucket with warm water and dissolve two pounds of powdered oxalic acid (obtainable from
paint stores) in it.
7. Using a Water Puppy
pump or equivalent, circulate the oxalic acid solution through the water intake hose, through the block, out the
exhaust outlet hose, and back into the bucket. Do this for at least two hours or until no more crud comes out of the block.
8. Empty the bucket and discard used solution safely.
9. Refill the bucket with two lbs. baking soda dissolved in 5 gallons of water. Circulate for at least 15 minutes.
10. Reconnect engine hoses, install thermostat and water impeller.