monoclineI think you're mistaking feathering with bleeding.
If something feathers, it means the ink tends to 'fuzz out' on the paper, you are right that feathering does not depend on GSM, it depends on how the paper has been sized and calendered.
If something 'bleeds through the paper', it means that you can write on one side, and then see what you have written through the other side.
68 GSM will absolutely bleed through. At that point the thinness of the paper is more like tissue paper.
alexandriao@alexandriao I beg to differ about Tomoe River paper and some other brands. It does not bleed with nearly any fountain pen ink and normal fountain pen writing pressure. Flex nibs are usually fine, too. There is some ghosting with 55gsm but 68gsm it is very little. Watercolors and alcohol-based inks like sharpie or sakura microperm etc. can bleed through almost everything and then 100-120ish gsm is necessary.