Just for emphasis, I again implore you to read:
The original 2002 UIAA tests that showed a possible 50% strength loss for ropes masked with felt tip pens, are briefly summarized in this document:http://www.sterlingrope.com/media/docum
... manual.pdf READ THE ENTIRE section with the caveat by Pit Shubert, of UIAA.
Again recall what the UIAA means by 50% reduction.
The above report also found that getting a rope wet could reduce "strength" by 70%. That is more of a worry to me, as ropes tend to get wet all along their lengths.
CClaude seems to be focusing on some extreme scenarios that could degrade a rope. We do know that sulfuric acid is a bad actor, but I don't think it will be present in commercial markers with a "0" MSDS health rating. I'm pretty sure I could detect creosol just by odor.
I'm somewhat amazed, given that I see extremely worn, fuzzy, highly discolored ropes being used in yeoman duty all the time. Our club simply retires and cuts up ropes that have more than 3 bad falls.
A lot of static ropes nowadays are polyester sheaths on Dyneema, which generally shows even less solvent effect than nylon.