Ok, end result, the best thing I used was mineral spirits with a soft brass wire brush as Max suggested
Caution to anyone else doing these, the maximum you want to use is a M8x1.25 and even then you will need to grind the head down to clear the height adjuster bar.
1.25 max pitch as the wall thickness is closer to 5mm so 1.5 is too coarse to get a few threads tapped in properly
