Use turps, white spirit or metho. You are probably getting the marks because the wet 'n' dry paper isn't very good and the grains are coming unstuck and embedding in the wood grain. I bought cheap wet 'n' dry paper myself from eBay and the same happened to me. I'd suggest trying to find some better stuff.
Last edited by Simon Barden; 04-02-2018 at 08:29 AM.