Go to mask, load from alpha channel, save to disk. Now you have an editable *.msk file, in grayscale of course.
When you're done with it, open your *.tga file, then go to mask, load from disk, *make layer invisible* in layer palette, save to alpha channel, save *.tga
Some *.tex files become corrupted, and they're not displaying alpha channels correctly. You can see on them that whatever you did on alpha channel, it always displays it in only black or white color (not grayscale), and there might be big black spots on car textures.
If this happens, you just have to use any other good *.tex file, import your *.tga in it, and continue your work with it.
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Bojan Tarticchio