Animated GIF has transparent background in each frame

Folks

I have created a 12 frame animation where each layer in PS has a transparent background - I know now I should have used a white background. In ImageReady the animation looks perfect but when I 'Save Optimized' and then play the anim gif back, the transparency is having the affect that the old frames are still showing when new frames are supposed to be hiding them. Hope I make sense.

How can I set the file so that the transparency is discarded?

I am baffled by this. Also, how do I change an existing layer in Photoshop that has been created with a transparent background to now have a white background?

Any help greatly appreciated.

Jump back to Photoshop and create a new bottom layer that is filled with white. When you jump back to ImageReady test the animation and it will probably include the white layer in each frame. If not, all you have to do is select each frame by hand and then make the white layer visible, one by one.