What paper stops feathering?

Coated, well-sized paper: Rhodia and Clairefontaine (90 gsm, satin-smooth), Tomoe River (impossibly thin, remarkably resistant), Leuchtturm and Midori for journals. The coating keeps ink sitting on the surface instead of wicking along fibers.

If you are stuck with office copy paper, go finer and drier: an EF or F nib with a well-behaved ink cuts feathering to nearly nothing without changing the paper at all.