Has anybody any thoughts on using something like this:

Veranda 4 in. x 4 in. x 100 in. Vinyl Fence Post Jacket White-73002149 - The Home Depot

and then filling them with sand or polymorphic sand or even concrete or even pee gravel?

You would put short pieces of 4x4 wood posts at the right heights to screw rails into.

Wouldn't that make a pretty much rot proof fence post?

 

The concrete is weak as a post and the PVC shell is also thin & weak. Might work if there was rebar in the concrete. Concrete filled posts work well when thick steel pipe is used as a shell.

 

but is there much point in filling thick metal pipe with concrete? My goal is to try to get a "rot" and or rust proof post...

 

Yes, filling a steel pipe with concrete does increase it's strength. They are two materials that work well together and you end up with something greater than it's two parts. Filling a vinyl sleeve with concrete will be stronger than just the sleeve on it's own but don't expect miracles. PVC is pliable/bendy and it expands and contracts considerably with temperature change where the concrete in the center will be very rigid and stable. The concrete will soon loose it's bond with the PVC and you'll have a concrete column and a PVC sleeve. Each will have their own strength but there isn't a great benefit to having them together. In your case the concrete core should be made strong enough (with rebar) to be the post on it's own right and the PVC is there just for cosmetics.

 

good info! makes a lot of sense! thanks!