Encrypting traffic between two public IPs

Hi All

Not really sure how to do this (or if its possible).

Essentially I want to ensure traffic (which is normally unencrypted) can be sent securely accross the internet. I can't create a "route based VPN" between the two sites. All I need to do is create a policy (I'm assuming IPSEC) between two public IPs, and tell the respective Fortigate's at each end to encrypt/decrypt any traffic which flows between those two points.

For example, public IPs 100.100.100.100 and 200.200.200.200

Fortigate 100D with FortiOS 5.2.5 at each end

We want to encrypt (with a preshared key) any traffic travelling between these two Firewalls public IPs irrespective of protocol etc. I thought this would be a Policy Based IPSEC, but I wasn't able to get it to work. I see traffic going out on the correct rule after setting Action = Ipsec, but no packets shown on the destination device.

Any advice on direction would be appreciated.