water-resistant, but hot water-soluble glue?

You could try Elmer's Glue-All in the school variety -- should hold long enough but will not firmly "burn" into either material and will easily scrape off.

Cookie Sewell

Thanks. Some of my parts have deep cavities/ holes, so, I can't really scrape the glue off. That's why I am thinking of using some detergent/ hot_water_solvent

While not technically a glue, you could use Friendly Plastic:

to hold the parts together. This stuff is similar to hot glue, but stiffer/harder at room temperature than hot glue. It softens at around

150F. One issue (which may be an advantage) is that it really sticks to almost any surface. You may have to scrape off small bits off the recovered parts, which will be an issue if you have very small, intricate or fragile details.

Another option might be a wax, like Flexwax (which is a reusable mold material that melts at 120F):

Or even just regular beeswax or a dental wax.

Frank

I am actually considering bee wax.

The perfect glue would be a glue that hardents at room temp, water- resistant, but soluble in a mild organic solvent like ethanol.

Super glue?

Hardens at room temperature. Is water resistant.

Can be disolved with acetone.

Of cousre, you may have issues with the acetone reacting with whatever crazy objects you're planning on glueing together.

I did not know Superglue was acetone-soluble. I will give it a try! I am trying to glue aluminum to glass.