I need to install a new mailbox. I will use a PT 4x4 sunk about 24" into the ground and the mailbox slips over the post. I am not certain what I want to use to secure the post. I could use a concrete mix or the polyurethane foam stuff. Does anyone have experience with the expanding foam? I am looking at 24" deep and realize the frost depth is over 36".
I would go with the concrete mix. Just do it like a fence post dig the hole dump the bag and pour some water in. I only used spray foam for insulation around windows or to fill a gap. Never tried it to support something.
I've never used it, but I would give it a try. I found several on Amazon, they all say it's as strong or stronger than concrete and definitely easier to use. Here's one
