Actually, I believe that was a common method in the heyday of PM.  The key to success was PRESSURE.  If you were to look at the presses that are used for papermaking, imagine stacks of paper within the press.  I've even heard that many times they didn't even use adhesive, just paper, water and pressure. Then it was cut with a saw like lumber, and turned.

Sue

Sounds a great idea,  lets face it we experiment all the time with this medium and the joy of it is that we can still recycle the disasters in the paper skip when it fails.

I love Papiermache!

I think I have an old square flower press somewhere, that may do the trick, especially with the addition of a large G clamp in the centre to apply even pressure.

The simplest form of press is two sheets of wood with four holes drilled.  One in each corner.  Then each hole has a screw through it with a wing nut at the top.  The only problem with this is that the pressure is applied from the edges (hence me postulating using a g clamp).

Sue's suggestion of an apple press is also a good one, they have a single screw down plate in the centre so that the pressure is applied evenly.

Of course if you are only flattening a few layers there is a good old fashioned method.  My mum and I used to do this to press flowers when I was little.

Put the piece to be kept flat in between sheets of paper and then lift up the carpet in an area where people walk through regularly.  Lay the sheets down and put back the carpet.  Leave until the you think the piece has had enough flattening and remove from under the carpet.  It worked a treat with flowers and we used to put them under the hall carpet by the front door.  Lots of people walking over them there.

I will let you know how I get on with the press.

I have been thinking this over and you are right with very thin sheets there is not enough edge exposed to allow air drying.  I would be inclined to use the press to see if you can pressure a fully dry (warped) sheet back into a flat sheet.  It might be worth investigating how paper makers get their sheets to dry flat ( I think they use a mesh ).

have to say that I am planning to try this to produce largish blocks of solid paper that I can then carve into puppet heads.  I am keen to get a grain finish similar to wood.  As I plan to aim for blocks of about 2- 4 inches depth air drying is not a problem.