That's the exact stuff I ordered. Any pics of your fence posts to see how it came out colour wise?I used barrettine on my fence - this stuff - https://www.toolstation.com/barrettine-wood-protective-treatment-5l/p48655
If you're using the same stuff, if you want an even finish, the bare wood needs to be stripped back to a somewhat consistent level before you start, as it doesn't "cover" the wood. Any darker patches will be darker after treating. Hope that helps.
I'd try to get the whole lot sanded back, or blasted, if you want it to look nice.
If you just want to protect it, you can just quickly brush over the whole lot and crack on - this would probably work best with a dark colour.
It's good stuff though and soaks in really well
EDIT - It hasn't half gone up in price.. I paid about half the current price couple of years ago!
Thanks RE: stripping back. I'll keep it as good as I am willing to get it consistently over the 40m2 lol... ah man... so much effort.