Hi,
Instead decals I used Pactra acrylic paints. Surface prepared with sandpaper (grit size 1200). I used 0 and 00 brushes.
Here is result.
As you can see I decided to paint "normal hand" on Echo's phase I armor.
My son is playing figures intensively for 5 days and there is no damage of paint.
I made this test on helmets originaly painted by Lego with some small erros (black and gray paint).