It sounds like you have pin holes in the hotcoat which is caused by dust settling into the hotcoat as it cures. When you sand the board the sanding dust gets caught up in these little pin holes. In the end you won't see the imperfections that much but it is best to blow out the dust with compressed air and follow up with a needle to scrape out what's left. Not worth the needle effort as far as I'm concerned, like I said you won't see most of them when glossed over.