How To Counter Decay?

1.) How do i repair items that are decaying?
2.) Lets say i take over an unfinished base that is showing decay. Ive finished it so half of it is now fresh and the other is decayed. How do i repair the decayed part of the structure that i may not have built?
3.)Do Spikes decay?

spikes do decay, put regular wood logs on your hot bar walk up to them and push the button for that hot bar slot. same goes for wood walls etc, metal I’ve never seen it decay yet because im on frequent enough I heard metal fragments are what you use.

You can repair anyone’s structure i believe, i repaired my friends wall spikes.

As far as the other persons base goes and stopping it from decaying im not sure how that works.

spikes decay over a couple days, but if you place them on a foundation that is connected to your main structure they are then linked to the decay timers of that building. Which get reset every time you interact with that building

If I hit large spikes from the side with my Hatchet will I take damage?