Usually we don't use sand paper.... But it's done so don't worry about it, but next time do this or at least that's what we use to do with racing XC skis when new and never used before, utility knife blade supported by a wooden block so the blade doesn't bow, and you very gently "scrape" the base of the ski from tip to tail. do a few passes! Then you wax with a really soft(warm) wax and when the wax is still warmish, you scrap it off using your plexiglass scraper. then you apply the wax you need based on the temperature you are expecting to have. Remember that if you use a really hard(cold) wax, don't wait for the wax to be totally cold before scraping, do it when the wax is slightly warm or still a bit soft, that way you don't raise "hairs" while scraping and also the wax doesn't fly away in a chatter of little pieces that always end up in you eyes....