Try this:
sudo edit .. /etc/pacman.d/gnupg/gpg.conf
no-greeting
no-permission-warning
lock-never
keyserver hkps://keyserver.ubuntu.com
keyserver-options timeout=20
keyserver-options import-clean
keyserver-options no-self-sigs-only
% sudo gpg --delete-keys 4B9100E679075EDF
% sudo gpg --recv-keys 4B9100E679075EDF
% sudo pacman-key --lsign-key 1E389882EFEABB1AE7723A014B9100E679075EDF
% sudo pacman-key --keyserver 'hkps://keyserver.ubuntu.com' --refresh-keys 1E389882EFEABB1AE7723A014B9100E679075EDF
% sudo pacman-key --populate obarun archlinux
% sudo pacman -Sy archlinux-keyring obarun-keyring
That's all the tricks I've discovered and tried.