So a while ago I was asked about a subject that is a constant problem when it comes to watercolour...and that is warping.
Anyone familiar with watercolour or any form of wet based media knows what I'm talking about and has the scars from attempting to claw your face off to prove it. It's unavoidable sometimes depending on how much water you used and it's such a shame when a piece you've been working on for a good length of time ends up with those ugly, bumpy areas that just can't be solved. It becomes a nightmare when it comes to scanning and then it leads to conducting a post mortem on photoshop.
Depending on how strong the medium you used and as long as it's just a wash or two,you can do something about it and there's a very handy process I like to use when it comes to flattening an image.
Once your image is completely dry, face up, lay two sheets of paper covering the entire image. Make sure that the bottom page is moist before applying the dry piece on top of it.
When that's done, place weight on the image using books,boxes etc. on the pages and leave it overnight.
If you did it correctly, your image should be completely flattened and ready to scan!
I heard you can also iron instead of the weight and it's quicker but I haven't tried that to be honest! (Recommend you read up on that before you try! )
This is a method I found online that I've used for the past while and it has served me well. If you know of any other techniques or alternative ways you approach this method, feel free to share below.