In this photo I used butyl tape as a bedding compound, it's what I use on 90% of my deck fittings. I loosened the nut and pushed the bolt up to show how much allowable movement can be had with a countersunk hole.


If you look close you'll see that the outer edges, where the bolt head meets flat deck, the sealant has begun to fail or failed entirely yet the butyl in the countersunk recess is still stretching and still there! The butyl is also still well adhered to the threads from being compressed by the bevel when tightening the bolt.