Mineral spirits only on spots like tar, otherwise just mild soap, period. I would not ,or ever use , lacquer thinners on my sails as it will have an adverse effect on it.
So tonight I sacrificed a perfectly good scrap of grape Icarex about 3x12". Made marks down the length with black sharpie, silver sharpie, ball point pen, pencil, and white fabric pencil.
I soaked one end with mineral spirits and scrubbed with a paper towel. Let it sit a minute then dried with paper towel. On examination, the mineral spirits had no noticeable effect. All the marks where still there and the fabric and coated looked fine. So while mineral spirits does not appear to harm Icarex poly, it is limited to cleaning tape glue residue and tar spots.
The other end was soaked and scrubbed with lacquer thinner. After a few seconds ALL the pen and pencil marks were removed. Rubbing again with a clean paper towel showed a little bit of purple dye on the towel. Hmmm. After air drying it is apparent the lacquer thinner took off a layer of the fabric coating. The fabric was no longer shiny but otherwise unharmed. Water did not bead as readily on the lacquer stripped end.
In conclusion mild soap and water is best. Spot clean with mineral spirits if necessary and live with any pen marks. DO NOT use lacquer thinner.
Did not have a piece of nylon handy but would expect similar results.
I would be interested if someone has way to safely remove pen marks from sail fabric.