Ford Repair: 99 Taurus heater core keeps plugging with rust, ooo miles, taurus wagon

Question
99 Taurus wagon, 3.0 V6.  I bought this car 3 years ago with 55,ooo miles.  I later discovered I could get no warm air from the heater.  A mechanic repaired it by replacing a plugged heater core.  Within 2 months it was plugged again and the mechanic replaced it free.  Within 4 months it plugged again so I just let it go for a year.  Two months ago I took it to another mechanic.  He found the core plugged with rust and was able to open it by back flushing it.  Today I took it back, plugged again.  He back flushed it and I now get heat, but it doesn't seem to be as warm as it should be.  The car currently has 125,000 miles.  I keep a 50/50 mix of green antifreeze in it.  The mechanic checked for a leaking head gasket and that seems to be fine.  Do you have any idea of what is generating so much rust and how to stop it or what I should do?  The first mechanic said he had this happen once in a Chevy and they had to replace the water pump because it was causing electrolysis.  Thank you for any help you can give.

Answer
google 99 taurus brown coolant and do some research and it may answer part of  your question-but if at any time you or previous owner didn't change coolant when it was due part of the blame is there also-replace the metal tubes coming from block to heater core-flush block and heater core good luck.