Why anyone would use a product who's bond to gelcoat
is actually stronger than the gelcoats bond to the substrate is beyond me. While 5200 is flexible compared to some other polyurethanes, 700-800% elongation before break, it is the tenacious adhesion that is generally the problem.
One thing I never hear mentioned is that Polyurethanes do not like UV yet I see folks use it constantly in UV exposed settings. If your polyurethane
bead is exposed it will and can eventually crack & fail.
Polyurethanes, according to a friend who is a chemist, generally continue to harden, though at a very, very slow rate, throughout it's life span. This curing or hardening of the product can reduce the elongation before break numbers over time and is a possible hypothesis as to why we see so many failures and leaks
Polysulfides on the other hand are not as affected by UV as polyurethanes and while their initial rated elongation before break numbers are lower than that of 5200 or even 4200 they should not diminish over time as polyurethanes tend to. Polysulfides, even years later, are generally as flexible as the day they finished curing where polyurethanes are not.
Let's take a seacock for a example. Using 5200 for them is kind of like welding on your lug nuts. Think about it. Lug nuts go down the road at 70-80 mph for thousands of miles on end, take massive pot hole hits, deal with poor alignments, bent rims & tires out of balance. Between rim removals they rarely if ever get tightened, torqued or looked at and still nobody glues them to the studs or puts a glue between rim and hub? They don't even use blue Loctite!
A proper seacock is fastened with large main threads and proper torque, and then through bolted to the hull
with multiple smaller bolts. A seacock does not spin, move or hit massive pot holes. Despite the shear lack of abuse they see, when compared to lug nuts & wheels, people still insist on "welding" them to the hull
with 5200! All you really need is an underwater rated & capable sealant
. Even 4200 is slightly over kill. Mechanically fastened hardware
is MECHANICALLY FASTENED!! All the sealant
has to do is be flexible and adhere to each surface enough to not part during any flexing.
My 30 year old boat is bedded, and was bedded from the factory, entirely with butyl tape. Buytl tape has about a 10 PSI adhesion but is MUCH, MUCH, MUCH more flexible than any polysulfide or polyurethane
. Over 80% of the fittings on my deck
, including the deck
to hull joint, are still bedded with the original butyl from 30 years ago!! It's still flexible and still NOT leaking.
You do not need to glue mechanically fastened hardware
to the deck but it does need to be flexible and seal out moisture butyl tape does this as do the lower adhesion polysulfides LifeCaulk & 3M 101.
You're looking at an elongation of well over 3000%. This butyl bedded cleat was removed at the 29 year mark, still BONE DRY and very, very flexible!
Was Still Bone Dry:
Popular Marine Sealants
3M 5200 Fast Cure
Boatlife Life-Seal (A Polyurethane / Silicone hybrid)
Sikaflex 291 LOT
Sikaflex 227 Fast Cure
Here's a quick run down with specs:
3M UV 4000
is a one part polyether with UV inhibitors added. It has a elongation before break of 800% and a tensile strength of 300 PSI. Spec wise UV 4000 is very similar to 4200 but because it is a polyether, and not a polyurethane, it reduces ti's elongation before break slightly. This should not be a concern if you use proper bedding techniques.
is a one part polyurethane with NO UV inhibitors added. It has a elongation before break of 900% and a tensile strength of 300 PSI.
is a one part polysulfide. Polysulfides have a natural UV resistance and are the premier product for use in teak
decks. Polyurethanes do not do well with UV so UV inhibitors are added. 3M 101 has a tensile strength of 139 PSI and an elongation before break of 416%. Life-Calk is a polysulfide rated at about 80 PSI adhesion.
is also a one part polyurethane (my personal favorite polyurethane) with lower adhesion than UV 4000 or 4200.. It offers an elongation at break of roughly 600% and is the easiest one part polyurethane sealant to get off in the future!
Here's one last photo
. It shows a cutaway view of a section of deck core
that has been potted with epoxy
and then countersunk to create an o-ring effect.
In a mechanically fastened joint, genny track, stanchion, traveler, deck cleat etc. you will NEVER, EVER see movement like this and if you do you have way more trouble than a failed sealant!! Remember this is a Butyl tape bond strength of about 10 PSI, not 1000 PSI, not even 100 PSI, just 10, which is nowhere near the 502 PSI of 5200 bonding to gelcoat
, or 362 PSI of 5200 bonding to stainless steel
Please don't get me going on silicone as I hate to even use it for the very few things that it is actually a proper choice for..