SOLVED. Childern's book about water-soluble spaceship [s]

[Vanessa]

Here is another book from his childhood that my husband asked me to post here :

A story about some boys who meet a strange little man who is very mysterious (I think he is a substitute schoolteacher). The man teaches them how to build a spaceship out of odds and ends, and then they make the spaceship airtight with a special paint, which seals the spaceship. They go to a strange planet, then when they come back to earth, they crash into the water. Water makes the spaceship dissolve into its component parts (I think the man told them not to get the paint wet). At the end of the story, the strange little man floats off into the sky, he is some kind of spore being.

[Heron]

Perhaps the 'My Teacher is an Alien" series by Bruce Coville?

[Vanessa]

Thanks for the suggestion - but we discovered on the Children's Books discussion group that it is "The Wonderful Trip to the Mushroom Planet".