Simply cut the tape to fit around the Hatches and this will allow you to abrade the surface without damaging the surrounding parts of the Tank body, How are you cutting out the Hatches? I use a step drill, they are not cheap and are dependent on a certain delicacy of use I would suggest a step drill above 28mm size about 30mm is about right, A tip, I would cut from the inside of the body outwards, do not push to hard as the plastic will melt and ruin the finish, once you have the hole too size you now need to lower the plastic immediately around the hole, the flatter the better the hatch will seat, but a small shoulder is no bad thing, this is where the Duck tape will pay off because a rasp or medium grit wet and dry paper is the way to make a seat for the hatch and with the tape protecting the surround you can put some effort in with out worrying about damaging your Tank body, Ain't this fun! shaun