Yup, I've had the problem.

1) Tried several types of sealer, both silicone and not.  All failed
   even after disassembly and careful cleanup.

2) Tried VW's special stuff   ...  priced like gold.  Didn't work.

3) Put in AlumaSeal.  That was several years and tens of thousands of
   miles ago.

Others have recommended Bars Leaks, especially for larger leaks.  I don't 
know, but I chose AlumaSeal because I reasoned that the extremely large 
surface area of all those aluminum particles might be sacrificial and 
corrode rather than the aluminum engine parts.

Mike