ZAMAC stands for Zinc Aluminium Metal Alloy Casting. The stuff our toycars our made of. Incase you are using caustic solution like sodium hydroxide, be very careful with the stuff. The chemical is highly corrosive and discolors anything that comes in contact with it. When you mix it in water (normally a 10% solution is good enough for such jobs), the reaction is exothermic (i.e. the solution gets very hot). Please use handgloves while handling the stuff and wash off anything that has come in contact with it in cold running water.
By guesstimates, for corrision of such kind as shown in those models, the part will have to sit in the caustic solution for atleast an hour's time or so.
All the best with your next venture cobalt. :cheers