Adding noise to the final image will fool the eye into not seeing the banding.
The issue can stem from a number of places:
1) it might not actually be in the file- with 8 bit monitors there are only so many versions of a color to go round. So if you have a blue gradient that crosses the entire screen and you are using the sRGB or adobeRGB gamut for your screen set up, there just aren’t that many versions of blue in that gamut to allow for each pixel across 1920 of them to get its own unique color. So you end up with, say, 10 pixels in a row getting the same exact tone and hue before the next hue gets put in on the next 10 pixels – voila! stripes!
So you can fiddle with your screen/gfx card colorspace or get a higher bit screen. But thankfully, when printed this may not show up if this is the cause.
2) same issue as above but the color space of the file is insufficient to allow enough colors. This becomes doubly (or trebley) true when you start gradating from one color to another. There are only so many cyans in a specific color space and only so many yellows etc so you can start getting what amounts to moires building up as these two stepping patterns become interference.
3) some effects (like glows) can compress the rate at which a grad spreads out and interacts with a grad below (when using a non “normal” transfer function)
4) The mask or clippingmask of a layer may not have enough bit depth to prevent that layer interacting, causing banding with the layers below. This goes double for images brought in that are used as masks or content – check what bit depth they are.
5) some layering transfer functions can cause the colors to roll around the “end” of the color spectrum/gamut and depending on the function can begin to clip. Culprits include the grouping that starts with Softlight and ends with Pinlight. Darken and lighten also have funny math that causes clipping or “compression” depending on whats below the layer – in these cases try screen, add or multiply – the worst these do is add up to white or black (the end of all clipping)
6) any compression put on the image at the end (jpg for instance) immediately reduces the color available to the file – see above for number of colors available problem.
7) make sure you aren’t looking at the preview proxy – its always horrible