Prim is used to people who have calluses on their hands and feet. She probably would have developed some sooner or later if she hadn't been so interested in being a doctor. She's always needed to keep her hands smooth so she can grip her equipment and help keep people calm. Here this is less of a concern, but she still has never been much of a worker in that sense.

"What?" she gasps, half a laugh of incredulity and half actual worry. "Is there room on your side?"

There's no point in asking if it's safe. It probably isn't, but she knows Tim wouldn't ask her to do anything outright dangerous. He's not like that. He wouldn't risk hurting her.