> Back prime and paint the

Back prime is a very short way to say "prime and pain the back side of anything you put up", I was specifically talking about priming and painting the back side trim itself in that case. You would also generally want to put at least one coat of paint on the front after installation to cover up the installation marks. I'd usually prime the whole wall and the front & back of the trim, give just the corner of the building and the backside/edges/end of the trim a coat of paint then re-prime the front of the trim after installation then paint over the whole thing. This is possibly overkill but it gets all of the surfaces nicely coated and is something resembling best practices.

For drainage and moisture prevention on the base you basically have two problems you're trying to solve.

First is to ensure that running water from like rain and snow melt will drain around the building and not into the building. So if the dirt up slope has slumped in along the foundation you'd want to dig it back a smidge and ideally grade it out to an even slope so it's less likely to slide in quickly. If it's loose dirt it's also probably a good idea to put something like some landscape fabric and bark chips or sod down to keep initial erosion from happening. This also means you should eyeball the overall drainage on the upslope side to ensure that there aren't any dams that water can get trapped behind. If the dirt isn't built up to where it'll block drainage and is stable then generally try to avoid disturbing it to much as that will likely increase future erosion (so it's kind of a trade off.. if there's a problem.. fix it.. but if there isn't then don't.. heh). Naturally compacted soil is usually fairly stable (moreso than most homeowner "compacted" anyway).

The second problem is vegetation and debris that can built up on top of the soil. This can/will wick moisture up and hold it in contact with the building and prevent it from drying out which will cause rot. This is often a problem around buildings where folks don't dig the down the dirt enough

One thing to remember about moisture is that it needs to be able to move. Ideally it moves to the outside of and away from the building. If you look at "good" (from an engineering perspective aesthetics are a separate problem heh) construction you'll start seeing this in all phases of the design from site prep to building shell design to window install to how exterior trim is designed and applied.

If you want some really interesting (interesting is I suppose relative.. heh) reading on building design https://buildingscience.com/ has a bunch of really fascinating articles and papers on building design and how they can go well or wrong. You can search for basic concepts there with google like "site:buildingscience.com back prime trim" (minus the quotes obvs).

Example relevant to the trim question: https://buildingscience.com/documents/building-science-insights-newsletters/bsi-015-stress-relief