The tattoo artist responsible for inking up almost the entire side of Aaron Carter's face claims he tried to talk him out of it, but the singer insisted ... and he actually wanted more.
Herchell Carrasco -- known as RockRollG -- tells TMZ ... he booked a tat session with Carter at his home Friday for $3k, but nothing really went according to plan. Herchell says he thought he was going to be adding more art to Aaron's torso -- there was no talk of a face tattoo -- but once Carter got the idea for Medusa ... everything snowballed.
Carrasco tells us Aaron knew he had experience doing face tattoos -- including some on Tekashi 6ix9ine's forehead shortly before he was arrested -- so Aaron got revved up for him to tat up his mug too ... RIGHT IN THE MIDDLE.
Herchell says he and Aaron settled for the side of the face, and he even tried to talk him out of that ... but he says Carter knew what he wanted and ultimately, it's up to the client what they want to do.
Carrasco adds, "Before I tattoo anyone, I take into consideration their mental state ... in Aaron’s case, I assessed the situation and I genuinely felt he was in a good place mentally to get tattooed."
We're told they initially used Rihanna's 2013 Medusa-inspired cover photo for GQ magazine as a guide for a stencil drawing, but then Aaron wanted the pic as his actual tattoo ... so Carrasco reluctantly obliged.
He tells us he did the large tattoo but Aaron wasn't satisfied -- he also wanted "Love" inked under his eye ... because he told Carrasco that's his brand right now.
After Herchell added a crescent moon tattoo on the other side of Aaron's face, he tells us he told Aaron he was done for the day. He says, "[Carter] wanted to keep going and I had to stop him. I couldn't cover his whole face in a tattoo. I just couldn't."
The good news -- for Carrasco -- he tells us Aaron ended up paying him $5k ... so a whopping 2 grand more than they agreed upon.
Also, he knows what people are thinking, and says Carter was only smoking cigarettes and weed throughout the day ... he didn't see him using any other drugs.
He says he was in a good state of mind the whole time, and as we reported ... Aaron looks like he's having fun with his fresh face of art.