How can I clean a lens (fisheye) scratches? I heard about
- peanut butter
- rubbing alcohol
- car paint scratch removal solution
- any other? like an abrasive tool etc
If the scratches don't affect the optical performance of the lens sufficiently to be perceptible in your photos the best course of action you could take is to do nothing. It takes a LOT of damage from scratches or fairly sizeable obstructions before they become noticeable! These photos from Roger Cicala's blog entry at lensrentals.com illustrate just how far a lens can be damages or obstructed with very little impact on image quality.
Anything you might do to try and make the scratches less noticeable when looking at the lens will likely do more harm, especially to the coatings on the outer surface of the lens, than good in terms of the lens' optical performance. And although it is true that the coatings on the surfaces of lens elements that face other lens elements are more important because they reduce the amount of flare generated by light bouncing back and forth between those two elements, the coatings on the front of your lens are there for a reason: to increase the amount of light that makes it through your lens to the camera's light box by decreasing the amount of light reflected off the front elements of the lens.
If the scratches are causing a perceptible effect on your images, then masking them with a very fine (to minimize the size of the mark) black marker can reduce the amount of flare to which the scratches may contribute.
There is really no way to remove scratches. You would destroy the optical coatings long before the scratch was gone.
Scratches on the front element have almost no effect on image quality, but you may get additional flare under certain lighting conditions.
Many people use a black marker to fill in the scratch so it becomes more flare resistant.
Scratches are not something that you "added", so you cannot "remove" it. Scratches, by definition, are small amounts of material, removed from the lens. So what you should to is "heal"/"rebuild" the lost material. And this is something that cannot be done at home, I believe. If you can wipe a scratch from the lens, it wasn't a scratch in the first place.
An important note - do not use alcohol or other solutions that are not meant to clean lenses. Lenses have special thin layers of coatings that improve their optical performance and protect them. Alcohol will just wipe them off.