Yevgeniy ,
There are no stupid question , just stupid answers do exist
The polyurethane resin I use is called "G4" and is produced by "Vosschemie".
This is the kind of resin used in the boating industry to coat the wooden hull before the fibreglass & polyester ( or epoxy ) resin laminations may be applied.
it comes straight out of the bottle , no need to stir with other products .
here the properties to be read on the bottle :
Now , this products yellows when exposed tu UV light , so I don't recommend its use on painted or printed paper .
There is another product existing in the Vosschemie range , it is called "G8" , it has the same properties as G4 but doesn't yellow at all , it is well twice expensive as G4 .
I presume those products are available at yours , with the Vosschemie brand , or maybe under another brand . The best option would be to go to a boating wharf near your home , or to a plastics specialist in the vicinity , to get that product or another one with the same qualities/properties .
it is sold in a one litre bottle , and costs about 17 � here near my home
I test my hulls in the bathtub , coated with G4 and left unpainted , I let them for a few hours , to see if they are properly coated , and till now , I never encountered big problems . The main problem I sometimes encounter is that some plating strakes were not properly glued , and water found its way between the layers , but the coated paper never shrinked nor has been damaged .
Ingura filmed my "Zinnia" this year , proof that card can be waterproofed without any problem :
http://fr.youtube.com/watch?v=4soKnbYRTtY
Anyway , here under a very good site with FAQs about cardboard building :
http://www.cardfaq.org/faq/
I know that some german modellers do sail paper models coming out from such a "book" , but I don't know which product they use for their hulls .
I know also that Jim Baumann is building card models too , I hope he reads this message to chime in .
Cheers ,
Laurent