The Federal Aviation Administration announced that effective February 25, 2019 they will require small drone owners to display the FAA issued registration number on the outside of the aircraft.
Before you use a permanent marker, consider using adhesive vinyl drone registration decals instead. Perhaps you will want to upgrade your UAS or sell your current aircraft. Selling it with your FAA issued number may be difficult if the number is handwritten on the drone permanently. The decals not only look great, they are easy to remove for when you are ready for that upgrade!
For less that $5.00, you can purchase your drone number sticker and install it directly to the aircrafts body making you fully compliant with the new FAA regulation. The drone decal stickers are sold as a 4 pack, so you will have plenty of stickers to install on your fleet of aircraft, or for that new upgrade that you have had your heart set on. Using vinyl stickers on your drone will make the number removable for future resale, they are professional, and very easy to install.