Red blotchy skin

Have some basic colour correction knowledge and often work on skin tone if editing other peoples footage but I am after some superior knowledge when it comes to fixing bad red blotchy skin in interview footage. Mainly in the neck area. I have raw footage in s-log 3 

Assuming you are grading in FCP, that can be a bit difficult. S-Log3 is not RAW, but it is a good log profile and we use it frequently. That will help in any adjustments you make. You can apply a feathered mask around the affected area and desaturate the redness, and then track that mask if there is significant movement throughout the clip. Another option we have used is to apply a skin smoother effect to the clip, and then use a mask with desaturation. This often helps as it minimizes the sharp contrast in the blotches and seems to blend in with the overall skin color. There are a few plugins if you google them for skin smoother and I believe a couple available thru FX Factory also.
We once had a severe case that nothing seemed to really help and we produced the interview in black and white and it was very much acceptable.

I use PaintX from Coremelt for a lot of digital makeup work, too.