Gradient transparency doesn't hide self backward polygons

Hello, I'm building a VR game and I need to display the hands but not the arm, and I don't want to have just "flying hands" like in Alyx, then I want to have a gradient transparency along the forearms.
I'm using the "Standard (specular setup)" shader with a semii transparent png, it works except that the backward polygons (not backface) are visible.
Can I correct the issue with that shader or must I use a custom one ?
I tried to enable zwrite and disable ztest like that :

m.SetInt("_ZWrite", 1);
m.DisableKeyword("_ALPHATEST_ON");

But that doesn't help.

Thanks