Paneling and Trim - paint problem with crown moulding (with picture)

I am trying to paint a joint on my MDF crown moulding. I sanded the area down to get the two pieces that I butt jointed together to be more even, then I tried to paint it with semi gloss, but I can see a darker area underneath the paint. I went out and got some Zinsser B-I-N shellac primer, I applied 3-4 coats of primer on the area, then put on 4 coats of semi gloss and I can still see the darker area. Any idea how to fix this? Ive posted a pic so you can see what I'm dealing with. Thanks

http://img523.imageshack.us/img523/3735/1003688.th.jpg (http://img523.imageshack.us/my.php?image=1003688.jpg)

Did you use some kind of glue on that butt joint?

I used a little No More Nails paneling and moulding construction adhesive, which is sandable and paintable. I also used it on all my other joints, and I am not having this problem on those, so I don't think it is related to that. It is confusing me. The darker area looks like where I sanded.

MDF doesn't take well to hard sanding. After the enamel dries, does the joint feel smooth?... trying to rule out a shadow or sheen difference.

When you applied the BIN, did you stir it well? Pigmented shellac will settle and will almost be a solid at the bottom of the can - it must be stirred up.

The joint does feel smooth, I think the piece on the left is a bit higher than the piece on the right, but not enough to justify the darker colour. On the left piece at the top near the joint, some of the darker area isnt even on the joint, but on the left hand side of the joint.
I shook up the can pretty good the first time I did it. I tried applying more primer today, and I used a stir stick to mix it up. I applied 2 more coats of BIN primer today, with no success. I can still see it as though I applied nothing. I am totally confused

"I am totally confused"

Me too!! Was the BIN runny? or is it more like paint consistency?
Generally pigmented shellacs have great stain blocking properties:confused:

The BIN was a little more runny than paint, its kinda liquidy, but I mixed it up pretty good with a stir stick, so it was well mixed. Im not sure why the primer isnt covering it. To me it looks like it would just need like one coat of paint, but when I apply either primer or paint, nothing happens, the spot is still there. I am going to try putting on a dark paint colour that I have left over from another room, and see if that covers it, then prime and paint back to semi gloss white if it works. Its driving me nuts

You might try contacting Zinnser - maybe you got a bad batch of primer.

well I put on 4 coats of a dark brown colour, and even it wont cover it up. I can still see it. I think if i painted this thing black it would still show up. I really dont know what to do now, id take it down but I cant, the whole thing doesnt make any sense, something should cover it up. I emailed Zinsser about it, so i'll see what they say, but I dont think any kind of paint or primer is going to work

Try moving your lights. A slight unevenness can cast quite a shadow when the light hits from it from the right angle.

yeah my guess is its a shadow too, im not sure how to fix it. I tried applying several layers of spackle to try and build up the area, it helps a little bit, but I can still see it. Anyone have any ideas? or am i kinda stuck with it? Its weird that nothing covers it. Ive tried caulking, spackle, the primer, paint, nothing seems to hide it. Very strange

If it's a shadow, shining a light on it will make it disappear.

well I have a big window in that room that lets in alot of light. I can see the mark only during the day, at night, I cant see it at all with my 5 light chandelier on. Its only during the day, and I cant block off the window, so Im not sure how to fix it. It looks a little better than it did after putting a million layers of spackle over it, and priming and painting again, but its still visible, which I dont get

Is the "texture" of the MDF different where you sanded? is it porous?

You might cover the area with spackling and then sand most of it off - just leaving it in the porous areas, prime and paint - hopefully that will take care of it.