Unless you can dry it out it will corrida with the white deposit you will have seen
Also it will expand with the heat and produce a poor fit for the piston
BTW when I say soft packing it's graphited yarn , I do not like silicon o rings as piston rings ,because unless you can hone the cylinder to a mirror finish and produce the grove to the o ring data tables for the use that you are doing it will not work long term
Soft packing has been used for years , as have CI rings ,but you will need to heat react them to get the spring correct
But I am sure others will chip in and give you a different opinion but my advice is based on my experience over 40 years
Stuart