"We leave the engine on the ground."

Wrong. You leave the power source on the ground. The engine, which is literally the paint in this instance, is still on the craft. But having the power-plant on Earth makes all the difference.

Sounds okay, provided they figure how to make the paint, and that it's a one-way trip.

If I can choose the crew, I wouldn't have it any other way.

This is a variation on an idea of Freeman Dyson's, about personal spacecraft. He illustrated it by a drawing of a Volkswagen sitting on a tank of water, which would be heated by laser beams from the ground, producing steam. This would be to escape earth's gravity, after which you could use very small rockets for manuvering.

Also, Niven's launching lasers. (Although I can't remember whether those were visible light or microwave.)

"You leave the power source on the ground."

No. The power source is the bonds holding the paint molecules together. The microwaves just provide the activation energy to start the reaction (ie, break the bonds). If you're going to get unnecessarily semantic, you should at least be right.