Obviously Not Polished by Mark
1. Why would they invite Bar-Lev into their home to shoot the documentary before the whole controversy was brought up by 60 minutes WITHOUT FINANCIAL COMPENSATION, if they had something serious like this to hide. It was obvious Bar-Lev would eventually seek footage of her painting. (read the Q&A with Bar-Lev called "The Unseen Hand" for more info)
2. We clearly see Mark's drawings in the basement of their house. There's no way he could have created half of the stuff Marla did. Especially if she gave him something "less polished" to work with. Do you really think he would go out of his way to turn one of her "less polished" paintings into the spectacular ones (bottom feeder, asian sun, etc.) instead of just making the art from scratch? Why would he need to start off with a base of random abstract drawings in order to turn it into something great? AND WHY WOULD HE NOT WANT THE CREDIT? But seriously, you can see his paintings in the basement and there's no doubt in my mind that he couldn't polish anything to end up like some of the paintings Marla did.
3. They clearly state twice on the DVD extra footage that Laura was way more artistically capable than Mark. They left it out of the movie because it would only be a deterrent to the plot, but if anyone should have the finger pointed at them it's Laura. Someone even pointed out on the board that the brush strokes in the dining room on the wall (which were done by Laura) seemed to resemble the ones on the more "polished" Marla paintings. And it's obvious Laura wouldn't manipulate Marla's work. If either parent were capable of manipulation it's Mark, but if either parent were capable of actually polishing them it would be Laura (even though it's obvious she couldn't either). So there's no reason to believe either parent "polished" the works.
4. They love their children (this is evident in many scenes in the film). They seem extremely adamant that they both had no hand in "polishing" her work, and Laura even opts to take a polygraph test which in the interview on the DVD extra footage, Bar-Lev, right after she says that, stops her and says no, that he wouldn't ever want to do that. By this point he can obviously see what he needed to see. He just wanted to find verbal or visual proof to display to the audience, even though he knows deep down there's no logical reason that Mark would polish Marla's works. He will never get this proof.
On a side note, I think Marla is much more aware of everything going on around her than people give her credit for. Such as the scene where she claims that her brother painted a painting (which I'm convinced Bar-Lev took completely out of context, I don't think she was saying he painted one of hers). Also near the end where she tells her dad to help her or tell her she's done. A lot of people will see this as her showing what her father always says or does when she paints, but I don't. It's just so sad that he reacts so awkwardly afterward, which only forwards this notion that he helps her. I think she says some of this stuff in order to test the waters. She knows what's happening and what her dad is being accused of (maybe not the severity of it) and she uses that in a joking manner sometimes. If you're not convinced that she knows why she's being filmed and you think shes just an innocent litte girl who's being exploited, go back and watch the film again and notice all the times that she looks directly at the camera (one strong moment of this is when Zane, her brother, is rocking the chair his dad is in, with Marla on his lap, while Mark is on the phone. She stares right into the camera as if she knows exactly whats going on and why). OR just take a look at the first scene in the film. This completely backs up my argument. The frame right before the opening credits proves that she's toying with the filmmaker.
Feel free to argue any of my points if you still think that Mark polished her works. I'd love to see an opposing viewpoint that contains actual logic instead of unfounded claims. But I just can't see one single reason why Mark would polish her works or invite Bar-Lev into his family if he did.
i am the voice inside your head, and i control you.