Well, I certainly wouldn't take the model to rainy Amsterdam...
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Just kidding. At the moment I really don't know how to seal the surface of coated card. I don't want to get the surface affected by a coat so that it becomes dull or even darker. But I will put it in a crate after I have wrapped it up in a plastic bag to protect it from dust. I have done so with my other models and they look so clean and fresh as if they had been built a few days ago (even though some of them are about five years old).
I heard from other people that there is a special spray suitable for sealing the surface of ordinary paper models. This procedure is recommended at places with high humidity, such as Brazil (and NL, I suppose :mrgreen: ). You also could try hairspray but I haven't tested that yet, so try this on a piece of scrap paper first.
Anyway, IMO the plans of a paper model are of higher importance than the model itself. Just imagine a flood or an accident (hungry dog
!) which causes severe damage to a model. Without the plans you don't have the possibility to repair or even replace the model.