I've used GIMP for a few days now. It has a lot of impressive features, some of which Photoshop didn't even have. But I was unable to find a very important feature that I use all the time for retouching.

Is there a way to sample all layers using the clone tool? I know I could just sample the main layer and then clone paint on to a new one. But sometimes I'd like to reference the main layer and the layers in between. And I'd rather not flatten the image because I wouldn't be able to make adjustments to the individual layers later to perfect the image.

Your help would be greatly appreciated.

If I understand you correctly, you want to select part of the main (base) layer and paste that part into tne next layer, and so on?

See section 3.2.4 of the Gimp Help manual, on the use of the Free Selection Tool, also called the Lasso Tool.

Then see the sections of the manual on copy/paste and layer creation.

I trust I don't insult you intelligence, but I don't know your level of experience with the Gimp.