I know this is an old post, but the other day I needed to make some changes to this model and was having the same troubles forming clean mess. so after trying a few different processes , I end up with a clean mesh. So I thought I might update this post :) hope Michael can verify if my procedure works or was just fluke.

quick recap... Have a solid I created from a surface, created by Zsurf. has 1000's of points and would not Boolean union cleanly with an other solid (shield shaped solid) .

http://moi3d.com/forum/messages.php?webtag=MOI&msg=5112.1

Now what I did was place the eagle on the shield in it final position.. I then cut the shield and eagle into quarters... using Boolean merge made 2 cuts one along Y the other long X .

this felt me with eagle and shield each cut in 4 pieces. I then boolean union between each piece of the eagle and shield , this felt me with 4 pieces top left, top right, bottom left, bottom right , then I union the two top pieces so MoI only needed to work along 1 edge... then union the bottom 2 pieces.. again only 1 edge in play... then union top and bottom together again only 1 edge in play.

This seemed to work well, I was left with a nice clean solid. no strange internal surfaces.... But from now on , If I need to make any other changes I need to use trim. ie when I wanted to add a cufflink post, I needed to trim the post on to the base . trying to boolean join resulted into a mess a gain :) So I'm guessing when I use trim, MoI is only having to deal with the contact between 2 surfaces and not the whole object. ?