Difference problem

I have a screwdriver and I’m trying to do a difference on it. So I made a long pole looking model and I subtracted it from the main model…then pressed <X> to remove the already selected objects, leaving me with the result. Now, I noticed that there was a black outline around and I didn’t know what that meant. And now I want to subtract from the resulting object, again…but blender won’t let me. I get an error saying that both meshes must be a closed mesh. WTF does that mean?

Someone please help me out.

You can check these threads, they will be helpful:

http://blenderartists.org/forum/showthread.php?t=67078

http://blenderartists.org/forum/showthread.php?t=69104

Also you can post screenshots of what you want to do and maybe someone will be able to help you with your model.

Thanks for the links. I now realise that my mesh may have tiny hole(s), which explains the error. So now how would I find it?

Preferably you should try to model your object without using booleans operations, as you can read from the previous posts they never works fine and create many problems.