I have a gate mounted to a 6x8 post where the center has rotted out of the post. I am looking to fill the center of the post, rather then replace it if I could. The post itself is still solid, just hollow so the mount bolt have nothing to connect to anymore. I am thinking to try the expanding post foam inside the post, but have not worked with it and I have no reference if it would be stong enough to hold a bolt.
The thing about the expanding foam is it may force you to get a new post anyway.
When that stuff expands, it can push cracks apart and if you have enough hollow in that center, it may be enough to provide decent compression power to the foam.
On the positive side, the foam will keep water from going in there, which is causing/exacerbating the damage.
Will foam provide support? Marginally. Bolts won’t get much added strength, but the entire post may flex a tiny bit less, assuming it doesn’t split during the curing period.
Silicone may work better, but I’m betting the quantity needed would surpass the cost of having someone replace it all.
You won’t be out much if it doesn’t work, but if it does end up splitting the wood, you may want to be prepared to replace it.
Most of the expanding foams are not waterproof or water resistant. You have to paint or caulk over them to prevent them from absorbing the water