Paper is a great and relatively inexpensive substrate to make art on. I've painted on paper using watercolor, acrylic and oil paint. Typically, I use watercolor paper that is 140 lb (300 gsm) or heavier. When I was working primarily with watercolor paint, I used mostly 100% cotton cold press paper. When using acrylic or oil paint, I use the same or occasionally tree pulp paper, as long as it is acid free.
Regardless of the paint I'm using, I often find that the paper warps and remains warped even when the paint has dried. I've tried sandwiching the paintings between heavy books for days but the warping remained.
Finally, I found an adequate solution.
I have used this method for watercolor paintings, acrylic paintings and oil paintings. All work equally well.
Note: While it may not be necessary to seal the painting with a varnish (spray, brush-on or otherwise) before using this method, use caution in the amount of water sprayed on the back if you have used any water soluble media (non-permanent watercolor paint, pastels, paint sticks, charcoal, graphite, etc). If too much water is used, it may rewet the media on the painted side of the paper.
2 sheets of clean white paper - like drawing paper or parchment paper - that are larger than your painting
spray bottle with water (the one I use has only ever contained water)
your painting on paper
rigid flat object to place on top of your art, it must be larger than your painting (a scrap of smooth shelving, the hardboard from the back of an old paper pad, cutting board)
heavy item that will fit on top of the flat object (a bucket of paint, stack of books, a brick)
Steps follow below the video.
Step 1: Prepare your flat surface (I use the wood table in my studio) - make sure it's clean and smooth!
Step 2: Lay one sheet of white paper on the surface.
Step 3: Lay your painting on top of the paper, paint side down.
Step 4: With your water bottle set to the finest mist, spray the back of your painting lightly (for a small piece, I give it 2 or 3 sprays).
Step 5: Lay the second sheet of white paper on top of the painting. Cover with the rigid flat object (I use a scrap of laminate flooring or an MDF shelf). Place the heavy object(s) on top.
Step 6: Wait for an hour or more depending on how saturated the paper was - if it isn't dry when you check it, reassemble the stack and let it sit longer.
If it still has major warps in it, try the process again if you suspect you didn't use enough water.
I've had great success with this method. The paper may not be as perfectly straight as it was before it was painted on, but I've found it removes warps and if anything, leaves only a slight bend which is usually flattened out if the painting is framed or mounted to another substrate like wood.
If you have any questions or comments, please leave them below.
I'd love to know if you have another method or have tried this one.