Plasticard is fairly soft and malleable. I also found 1mm sheets to warp easily when I used it. For parts that need more stability I used 1.5mm, but I think you want to up thickness to 2mm to proof it against warping at least somewhat.
I'm not familiar with the Tamiya putty you used, but if it dries without cracking and holds together on its own, you might try applying it to a surface from which it easily peels and then glue it to your plasticard sheet in a second step. It's a little more work, but that will sort out the warping problem.
The plastic sheet remains malleable even though the putty warped it. Depending on how the putty reacts to cuts, you might be able to save your work by scoring the putty between the trees and cracking it by bending the plastic sheet back into shape. You want to be careful with the latter so as not to snap it, and of course not cut so deep that you damage the plastic sheet. Depending on how malleable the putty is, you might get a level base again. From there it's just a matter of filling the cracks, which shouldn't cause any more warping.
|