Credit: Michael Buckner/Getty Images; David McNew/Getty Images

Lindsay Lohan says her F-word court manicure was just a joke -- but her estranged dad Michael isn't laughing.

"It is stupid, it's insane, it's so dark," he said Thursday in an interview on FOX News Channel's Studio B with Shepard Smith. "Why would you do that, for what reason -- and not expect people to see it or word get out?"

The actress, 24, must surrender July 20 to begin her 90-day jail sentence.

"It's just tears me up," said Michael. "Especially to see that my wife, my ex-wife, is around her and not doing a damn thing to help her.

"I don't believe the jail poll was warranted," he went on. "There's measures she can take now that can put her in a better place if she goes to jail, or even help her not go to jail."

Michael insists Lindsay is addicted to prescription drugs (she denies it).

He and his attorney are even drafting a letter asking the court to prevent Lindsay from taking prescription drugs to jail with her.

"She should be in a rehab now," he says. "And what Dina should be doing, and I'm calling her out right now, is she should get the names of the doctors off every prescription, every bottle, and she should turn those names over to the authorities for investigation."