Is there anything that can make thin layers or pieces of cardboard hard as a rock or metal beside poly/epoxy resin?

I want to make the cardboard as hard and unbendable as possible.

With concrete?

With foam coat?

What else is there? 

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steveastrouk3 months ago

I'm failing to see how there can be a better answer than the one you've been given.

You aren't going to get decent results from smearing concrete on cardboard. Resin is very inexpensive.

Jack A Lopez3 months ago

It is kind of curious that you wrote, "hard as rock". It made me think of petrified (i.e. rock-a-fied) wood.

https://en.wikipedia.org/wiki/Petrified_wood

The last two references in the Wikipedia article for "Petrified wood", are to articles about an artificial process for doing this.

https://phys.org/news/2005-01-petrified-wood-days....

http://www.livescience.com/110-presto-instant-petr...

I am guessing you probably do not have an inert-atmosphere furnace that can reach 1400 C in temperature.

However, you did ask, "What else is there?"

steveastrouk3 months ago

Molten wax willl stiffen it as it hardens. If you keep the piece in a freezer it would be brittle.

Kiteman3 months ago

You could experiment with super-glue, pour it on the paper, except for the corner where you're holding/hanging it while it dries.

Superglue is horrible stuff to breath around. You'd need a full blown chemical mask to work near it.

rickharris3 months ago

As hard as possible isn't really much of a specification to work with. You already exclude the best alternatives.

In general with cardboard your better off making it strong with your construction technique rather than some coating.

However I guess you could use the cardboard as a framework to build concrete on to. This will be very heavy though and will need further reinforcement if it is of any size.

Foam will be brittle.

Pain will give some stuiffness and water proof the cardboard.