Re: What is waterproof paper coated with to make it waterproof?

Thanks for the question, Kat.  Waterproofing of paper extends far beyond 
the underwater writing paper you used.  Cardboard milk and juice cartons 
need to repel water (or milk or juice) and even normal writing paper is 
able to control the amount of liquid absorption so the ink does not run and 
smudge.  

Sizing is papermaking jargon for the ability of the paper to hold out 
water.  There are two main types of sizing: internal sizing, which is added 
to the pulp fiber when the paper is being made, and surface sizing that is 
added to the surface of the sheet of paper.  

Wood fibers are hydrophilic materials, meaning they like water.  Therefore 
making waterproof paper requires adding something to the fibers to increase 
their hydrophobicity (water hating).  These additives are known as sizing. 
 The type of sizing used depends on the conditions used in the papermaking 
process (acid, neutral, or alkaline).  In general, a hydrophobic chemical 
is bonded to the wood fibers and after the paper is formed, it repels the 
water that comes into contact with the paper.  This type of sizing can 
range from waxes to modified tree resins to low-molecular weight carbon 
polymers.  

The additives used in surface sizing are very similar to those used in 
internal sizing but the application method is different.  Here, an aqueous 
solution of the sizing is impregnated into the paper similarly to a coating 
operation.  The paper is then dried to remove the water it picked up with 
the surface size.  

I would imagine that your underwater paper was made with a combination of 
internal and surface sizing to ensure the proper water holdout was 
attained.  I hope this answered your question because sizing of paper is a 
fairly extensive topic.