backing plate for new thru hulls
You'd better call Groco and tell them to stop recommending 5200 for thru-hulls
Many companies recommend things just because they are known not because they are the best choice. I would not personally use 5200 on a backing block although I make mine out of solid fiberglass, which never rot, and I permanently glass them to the hull. 5200 is as permanent as fiberglass and actually has a higher bond than the gelcoat does to the resin..
Use a lower bonding agent like 4200 or Sikaflex 291 and when your backing plates get soggy you'll be happy you did. Better yet make them out of fiberglass and they will last forever..
P.S. You must be reading a different set of Groco instructions than I have seen:
Groco Instructions
As for Flex Bond I see no reason to use a product that won't disclose the following:
1) PSI bond strength??
2) What is it?? Polyurethane?, Polysulfide?, Polyether? Hybrid?
I personally don't like Tony the Tiger secret recipe products that won't even tell you the basic chemical make up or a class eg: polysulfide..
As for in-line they mean in-line with a thru-hull not a flanged adapter as the flanged adapter gives the ability to through bolt the flange and it matches all the threads compatibly.