With Por-15 I think its less to do with a smooth surface and more to do with not being completely clean. My engine block was the first item I used Por-15 with and I had to redo around the oil gallery plugs as there was some grease that leaked out which resulted in Por-15 coming off in patches. Once I recleaned and repainted it was fine.
I had no issue with Por-15 sticking to my floors and they were pretty smooth (though roughed up with 240 grit)
But I spent weeks getting where I thought they were clean enough for por-15. Then used boiling hot water to reduce the Marine clean and a stiff wire brush several times to get any remaining grease off. In my experience you have to have the surface crazy clean or don't bother, since it will just come off in sheets.
And you have to topcoat when your finger still drags on the por-15 otherwise you need to reprep the por15 surface so the topcoat will stick.