It's not the proper way to do it.

What you do is use nodraw on ALL decals. And then once your map is near completion you go round and create invisible walls where ever needed. So you would a brush covering your fence made of player_clip, clip_missle etc anything you need. That way decals use the decal texture, clips use the clip texture and nothing clashes.

Your directions for the fence make sense now that you explain it this way. My bad for missing a step. Thanks Techno!