Only got my MP20 HMB irons 7,8,9 & PW on the 28th July and have only played 6 rounds with them..!
so far the polished chrome finish is holding up well and is not discoloured in anyway,
one reason for the lack of scuff marks on the sole of my irons could be that I tend to Be a sweeper of the ball
and seldom if ever take a divot, I also wipe each iron clean after a shot and once home rinse under a look worm
tap drying them with a soft cloth, and I never use any type of abrasive cleaners on them,
Would be helpful if you could post some photos of the discolouration that you have on your irons so we could get a better idea of just what it looks like..?
Cheers
John