I tried timber formwork with a sand bund on the lower shed columns, and I found the formwork liked to float.
This time I mixed up some silver sand and rapid set to make the moats with a gauging trowel using a peice of timber as the former. Didn’t take any longer than making wooden formwork.
I assume blackjack you mean bitumen paint ?
I hadn’t planned on it as posts are galvanised, Bottom 200mm or so will be in the soil, then there will be a blinding layer / DPM / Insulation and poured concrete.
Now you’ve mentioned it .. I have gone off and googled it . probably will paint as well upto FFL internally.
Worrying now about where I have cast the panels into the webs of the steels on the lower shed 🫣. With no paint 🫣
As the thread title says - only half a clue