Damn, I should have seen this thread earlier! I *just* installed my B&W Companion slider & had the same problem - wouldn't sit down all the way due to corrosion in the turnover ball and the heavy powder coat on the post.
I was installing it by myself in my driveway, so once I started I just kept going. I jumped up & down on it, whacked it with a mallet over & over again, and got it most of the way down myself. Then the guys in the shop finished it for me with a sledge & 6x6 block of wood :)
I don't think this sucker's ever coming out, but if it does I'm going to save the tips on the first page and do the sanding & lubing next time! :-)