After about 16 hours the part seemed to have a good coating of nickel, allthough it looked dull instead of shiny. During the plating process I checked the temperature of the solution a few tomes and it was only 21 degrees. Recommended temp is between 20 and 50 degrees (C) so obviously the little 50 Watt heater that came with the kit was not strong enough. In the 2 pics above the higher hubcover is the one that has been nickleplated, in the first picture you can see the dull finish as it came out of the tank, in the second pic the part has been given a quick polishing. Hmmmm, not bad actually!
I mounted the part on the nickle plated front wheel and the colour matched the rest perfectly!

Before plating the second hubcover I first went to the petstore (?!?!) and bought a 200 Watt aquarium heater and an aquarium thermometer. Using both heaters the plating solution reached a temperature of 32 degrees C, and the part was fully plated in about 6 hours. It needed less polishing than the first part, definitely an improvement!