yeah on somehting like a toilet roll there is a gap between the two edges of the paper spiral as they wind up the tube. on the pringles tube those edges overlap leaving a thicker ridge as ben says.
The gel medium i used covers it well especially if you put it on thick like i have for a very rustic look. However i didn't plan it too well as i have a seem running right across the door which is only painted. I know its there and can see it, but i bet ya can't see it in the pictures
http://lh6.ggpht.com/_RRKBhA-2I4w/S_0KbO1QnHI/AAAAAAAACi8/dvx2j59dNzg/tower%20ebay.jpg
