stepping of tones photoshop

[sam joyce]

I have done some posters st 300 dpi - cmyk, with large outer glow effects to them which looks great on screen but when it prints there is a stepping effect through the changing tones of the gradient.Does anyone know what causes this?

[uk2usadaz]

You mean banding.
You don't see the banding on the monitor because your monitor is probably capable of rendering thousands (if not millions) of colors.
Solving banding has been bothering printers for years -- Short gradients are usually okay because they don't cover long expanses of surface. Full page gradients present a challenge because from 0% to 100% of any given color generates fewer than 255 individual colors -- banding will occur after the stretch goes over 8 inches. If the two colors in the gradient are closer in spectrum (gray to white, for instance) then even fewer tones are generated.

The solution usually involves adding noise to the gradient.
You could also try to do a bit of a guassian blur on the gradient.
Blurring smooths out the banding nicely and a wee amount of noise 2-3 level (normally monochromatic).