When testing methods to flatten sketches that had 
gotten wet, I tested ironing one. (Actually, I 
was aiming to find a way to restore to flatness 
stuck sketches that had been removed from their 
cels using the "water technique".)

I will FIRST say that it did not work well.

That said, I achieved best results when the sketch
still had a tad of dampness in it. I placed the 
sketch on a perfectly flat surface (I used the 
1.5' square marble tile I keep for folding 
origami, the likes of which can be purchased at 
home improvement stores in the tiling department) 
with at least two blank pieces of paper between 
the tile and the sketch, plus another two or more 
blank pieces between the sketch and the iron.

I did test this setup with a towel between the 
paper pile and the iron, but the towel seemed to 
have no effect other than make things take longer.

In the end, there were still wrinkles in the 
sketch paper, but not quite as many as in the 
paper I didn't iron. The big difference was that 
the paper I had ironed had gained a couple harder 
creases (in the wrinkled part) in the process, 
whereas the paper allowed to just dry, had softer 
waves in the wrinkled part. The softer waves 
don't attract the eye as much, and I determined 
them to be more desirable than the slightly 
increased flatness of the ironed paper.

Now everyone knows were a hell of a lot of the 
blank sketch papers I get with my AIC cels went 
to. ^_^

Many Sharp Smiles,
--Drac