I just finished a 2 color halftone poster, and I got a screen interference moire in both layers.

Here's the print and some detail showing the moire:

moire3 | Flickr - Photo Sharing!

moire2 | Flickr - Photo Sharing!

moire1 | Flickr - Photo Sharing!

I printed with 230 mesh, 40lpi, 15 and 30 degrees, respectively.

The faint ghost striping showed up in each burn (it wasn't in the films).
I'm using thicker vellums than usual from a different supplier.
I double wet coated the bottom of the screen only. Didn't coat the top side.
There was no moire created between the two layers.

Could the problem be one of the following?

1. Not enough emulsion. (Next time double coat both sides.)
2. Underexposed. (Next time increase exposure time.)
3. 40lpi is somehow interfering with the 230 mesh for some strange mathematical reason. (Next time change the lpi to 52 or some other number.)
4. Screen angles bad juju. (Next time try different angles, like 37 and 67)

Or all of the above.

Any help would be very - helpful.
Thanks!