Can you cover existing metal crowns with porcelain or another product to make them white?

Trying to mask an existing metal crown will be difficult and not long lasting.  It is better to have it redone with an all porcelain crown so that there is no metal under-structure.  Hope that helps!  Good Luck!    

Porcelain is baked onto metal at a very high temperature during the process.  The only way to make the crown match your teeth is to replace the crown with a porcelain fused to metal, all ceramic or zirconia crown.