The Human Centipede II

In the tollbooth of a multistory carpark, Martin Lomax (Laurence R. Harvey) is watching The Human Centipede (First Sequence) on his laptop, a film he is obsessed with. Overweight, asthmatic, and mentally challenged, Martin lives with his overbearing mother (Vivien Bridson), who blames him for having his father put in prison for physically and sexually abusing Martin when he was a boy. Dr. Sebring (Bill Hutchens), Martin's psychiatrist, also touches him inappropriately and prescribes him heavy medication. Martin keeps a pet centipede, which he gleefully feeds insects to.
Martin acquires a dingy warehouse after killing the lessor and begins abducting people to use for his own human centipede. His victims include: an aggressive young man (Lucas Hansen) and his girlfriend (Emma Lock), two drunk girls (Kandice Caine and Georgia Goodrick) who catch Martin masturbating with sandpaper, a man (Daniel Jude Gennis) and his pregnant wife (this couple has a toddler, whom Martin leaves in the back seat of their car uninjured), and another man. Martin also catches Dr. Sebring and a cabbie (Gabe Kerr) having sex with a prostitute (Maddi Black), and proceeds to kill Sebring before abducting the cabbie and prostitute. When Martin's mother finds and destroys his scrapbook, Martin kills her by bludgeoning her head repeatedly with his crowbar until her skull caves in. He then lures his detested neighbour (Lee Nicholas Harris) to the scene, shoots, and kidnaps him. Martin's final victim is Ashlynn Yennie, the actress who played "Jenny" in the first film (now playing herself, having been lured by Martin under the pretense of a film audition with Quentin Tarantino).
With twelve victims, Martin begins assembling his "centipede". He severs the tendons in each person's knees to prevent them from fleeing and uses a hammer to knock out their teeth. He slices open the buttocks of one of his victims, causing him to bleed to death. In a quick change of plan, instead of actual surgeon tools, he uses a staple gun and duct tape to attach each person's lips to the next person's buttocks. During the assembly process, the pregnant woman is presumed dead; a grieving Martin places her in the corner. His "human centipede" is ultimately ten people long with Ashlynn in front.
After performing the crude procedure, Martin begins experimenting. Disturbed by Ashlynn's screams, he tears her tongue out with pliers. He then injects each victim with a syringe of laxative, forcing each of them to explosively evacuate their bowels into the mouth of the person behind them. After pausing, he then wraps his genitals in barbed wire and rapes the woman at the end of his "centipede". As he finishes, the pregnant woman awakes and runs outside screaming, apparently in labour. She leaps into a victim's car and births her child. She then gets onto the road, but as she starts the engine, she stomps on the accelerator, crushing the baby's skull under the pedal. She still manages to escape.
The neighbour rips his face from the person in front of him, separating the "centipede" into two halves. Furious that his centipede is ruined, Martin shoots all the victims, and when he runs out of ammunition, uses a knife to slit the throats of the remaining victims. As he advances on Ashlynn, he appears to give pause, and kneels in front of her. She punches him in the genitals, shoves the funnel into his rectum, and drops his pet centipede into it. In agony, Martin stabs her in the neck and staggers out. Ashlynn is seen slightly moving afterwards.
The scene cuts back to the tollbooth, with Martin watching the credits of First Sequence on his laptop, with exactly the same reaction as the initial scene. However, the toddler left in the car from the previous kidnappings can be heard crying in the background.

Question: What do the drunk girls catch Martin masturbating with?
Answer: Sandpaper