[gentoo-user] Prefer Gentoo repository rather than overlay?

[Matt Connell (Gmail)]

Looking for some guidance in managing the source of package 
installs/upgrades when a package is provided by both the standard 
repository and an overlay.

I currently have the poly-c overlay added via layman.  poly-c provides 
many of the same packages as the standard gentoo repository.

When I install/update packages, portage appears to prefer the version 
provided by poly-c rather than the version provided by the gentoo 
repository, if the two provide the same version number of the package. 
Examples of this include sys-boot/grub, sys-fs/udev, and other critical 
packages.

I would prefer that portage prioritize gentoo's version rather than the 
overlay's version, unless specified otherwise (eg. 
sys-boot/grub::poly-c) when installing.

What's the best way to go about doing this?  I could specify ::gentoo 
for each entry in my world file, but this seems rather heavy-handed and 
high maintenance.  Is there a better way?