DON'T use Grez-Off to clean the engine!

I cleaned the 328's engine today, used Grez-off which i have used for engine cleaning on other vehicles. It did a good job on the engine per se BUT it spotted the paint on the coolant overflow tank. I have no idea why. It did NOT dry on anything, I hosed it off thoroughly with plenty of water. Otherwise the engine looks good. The paint on the tank was not removed or damaged other than the spotting. I tried cleaning the tank afterward with a variety of cleaners but nothing eliminates the spots. Interestingly, it did not affect any other black paint in the engine compartment - just the paint on the overflow tank. Maybe the tanks are painted with something different that reacts with the de-greaser.