Dissolved putty is great! (I tried to add lacquer thinner to the red putty I use but results were not so good.)
The idea of building this airplane from existing design elements is one of the strangest USAAF projects. My personal thinking is that General Motors proposed this because they had sunk a good deal of time and effort into the Allison 3420 engine and needed an airplane to use it in. GM was removed from the B-29 production scheme (that slot was given to Martin/Omaha) so that the P-75 project could be realized by GM. Regardless, it is an interesting machine!!!