Scott Yes..
Sikaflex bonds quite well to stainless and has double the flexibility for expansion/contraction between dissimilar products. I have never known 5200 to not hold well to stainless but I have seen it not be flexible enough for joints like stanchions, keels, dodger fittings etc.. If 3M says it only bonds to stainless at 10% than I guess it must be true but from my experience I've always had to use a wire brush on a drill to get it of a stanchion base or other fitting and a heat gun. I think the MOST important aspect is not how well either product bonds but how much elongation of a joint you can before sealant failure and Sika is TWICE as flexible as the 3M products. I use Sikaflex for my stanchions seeing as they flex more than anything else and considering how people grab them and yank on them all the time it is a perfect application. I have yet to have a fitting bedded with Sikaflex leak. It's not to say they eventually won't but I get more life out of Sikaflex 291 LOT than I do any other product. I'm kind of anal about re-bedding, so much so, that every fitting on my 2005 Catalina 310 has already been done, as a safe guard, mostly because I don't trust factory working hacks who could care less about the way a fitting was bedded! It's not their boat so all it has to do is pass the "squeeze out" visual test for final inspection and they are happy. Sikaflex 291 LOT still bonds quite well so I don't use it on plastic stuff or thin aluminum like a window frame but for just about everything else I use it. I still use 101 too and occasionally UV4000 but 4200, Silicone & 5200 are verboten on my boat. The keel on my old Catalina 30 was re-set with Sika about 10 years ago and to this day it's still bone dry even after the current owner admitted he's touched bottom a couple of times. The 5200 on that joint lasted 7 years... The trick to any joint remaining water tight is prep. Part of the reason I re-bedded my 2005 was so I could counter sink each hole slightly creating a sealant o-ring around the bolt and creating a gasket/o-ring type seal with MORE material/sealant to flex. The old sea tale of letting the sealant cure, before final tightening of the bolts, to create a gasket, is very dumb and I honestly can't believe many so called reputable authors suggest this. Think about it? You set a stanchion atop a blob of sealant and tighten the bolts slightly. You leave, let the sealant cure and come back in a few days to tighten the fitting. Great you now have a gasket but you just broke the MOST IMPORTANT seal of all! The seal between the bolt shaft and the sealant is the MOST critical because it is a direct path to the deck core allowing water to migrate down around the bolt threads and into the deck core. Once a fitting is bedded and cured a bolt or screw should NEVER be turned unless you are planning on re-bedding it again! I have seen to many surveys where owners used the "gasket" method and had wet core everywhere there was a bolt hole in the deck! I will not even look at a boat where I can tell this method was used to bed hardware.. Once a stanchion is bolted down you're lucky to have 1/32 of an inch of sealant left behind. Even a 700% flex is minimally sufficien, with so little sealant, so a small counter sink is the trick. You must also clean each surface with MEK or Acetone before applying the sealant and rough up stainless taking any shine off the base of a fitting. If the piece was previously bedded with silicone GOOD LUCK getting it free and clean. I use good old Scotch Brite and abrasives for removing Silicone unfortunately chemicals won't touch it as it is designed to be impervious to most all chemicals!
