cleaning used heatercore

I purchased an used wc kit and found that the heatercore is corroded with a white powdery substance. What should i use to get rid of this stuff?

CLR

you can use CLR.

thanks, i'll try it.

I'm curious, what components make up this kit? And what liquid mix was ran through it previously?

Ummm, I would avoid CLR on copper and alluminum, it says it will corrode some metals.
Here is a great article for cleaning heater cores
http://www.overclockers.com/tips250/

If you know of any clock shops in your area, take the core to them and see if they will run it through their ultrasonic cleaner ( give it a general cleanup first- they don't want to put a filthy part in their tanks...).
Make sure you flush it out well afterwords.

Ultrasound will dislodge even the most tenacious crud and the solutions they use are designed for use on brass so you don't have to worry about acid eating away at the base metal...

Depending on the makeup of the unit discoloration may occur, but some Scotchbrite pads and elbow grease will take care of that.