Here is a tutorial how to make decals "dissapear"
1. Make the image for the decals all black in MS Paint and save it as a 24-bit .bmp.
2. In TXD workshop click the button in the bottom right labeled "Alpha/mask" and select the decals image.
3. Click the Properties button in the top right and in there check the box that says "Alpha is used".
4. Replace the original decals image (Be sure you have Alpha/mask selected) with the all black image you made.
Done
This tutorial was posted by Stumpy in another topic , i just copied it .