Having followed this thread for a while I feel a need to clarify that there appear to be two separate rash issues. One involves a rash around the wrist where the band touches the skin and appears to be caused by the stock "plastic" band Samsung ships with the watch. The solution to that issue is to get a different band; leather, metal and some fabric replacements are the best. I bought a metal band at the same time I bought the watch so I never experienced this rash problem.
The second issue involves a rash or the appearance of burn marks directly under the watch which is what I have experienced. In my case it looks like little bumps surrounded by redness. Based on others' remarks this may be caused by some substance left over on the back of the watch from the manufacturing process (grease maybe?) or/and it might be caused by the green led actually burning or irritating the skin. I have washed the back of the watch with alchohol wipes but this hasn't solved the problem for me. I am contemplating putting a piece of tape (opaque or semi-transparent) over the green led but not sure what tape will be good as some tapes will likely irritate the skin as well.
Related to the second issue I have tried turning off the green led in settings related to activity and Samsung Health but I still see it flash on sometimes. Any thoughts on the right type of tape would be appreciated.
Interesting, I have exactly the same issue with a 2 week old 2019 46mm Galaxy Watch.
There was an issue with the bands for Fitbits that gave my wife a rash exactly the same and they had to recall the whole ***** lot.
I've just logged a ticket with Samsung Australia and will possibly hat a refund on the watch.