Polysulfide vs Polyurethane - Below the Waterline

[Texas Boater]

I started doing some research and now I am confused all over again!

From the data provided on all the marine sealant/adhesive manufacturer’s websites it appears that a POLYSULFIDE would be the best if not only logical choice for bedding the outer bottom planks to the inner plywood bottom of my CC. But when I read advice from ship/boat-wrights they all seem to favor POLYURETHANE (if they favor a modern chemical sealant in this location at all).

So what gives – why use polyurethane that will bond the outer planks tenaciously to the inner plywood and not provide as flexible a sealant as polysulfide that is more flexible and appears to be easier to remove for future repairs?

I could not find any scientific study of sealant properties of polysulfide versus polyurethane for use below the water line when subjected to the pressure, vibration and impact loading encountered by a typical boat bottom.

So what am I overlooking? Why does it seem that professionals prefer polyurethane (namely 3M 5200)?

[pipefitter]

I have seen polyurethane used under the waterline on wood boats. It seperates from the wood and then traps water in there. You can literally pull it out a solid piece of dirty yet still stretchy plastic. It sticks at first but doesn't stay that way. Might as well have used silicone. Urethane adhesive sealants work best as a gasket between fastened disimilar components such as metal fittings to wood or fibreglass. But if the screws aren't kept tight,the parts will become loose and seperate and leak. We use 5200 to bed our aluminum products to FRP boats. In the first few months after cure,if you try to remove the parts,it will sometimes bring gel coat with it. Left for a couple years and it will come apart without being stuck.

[Figment]

Are you looking for a sealant or an adhesive?
Polysulfide is a sealant.
Polyurethane is an adhesive (that just happens to be water resistant).

(some) Professionals prefer 5200 for two reasons.
1. Their average customer believes it's the best thing to use for anything.
2. It takes less time/labor to glue things together with 5200 than to properly fasten things together.

I don't understand how anyone who has ever undertaken a repair that involved undoing polyurethane sealant could ever consider using it again. In the long run the stuff makes far more work than it saves. Evil.

[Texas Boater]

So, from your two replies my research seems to be on track. 3M 5200 should not be used as a bedding material on the bottom of wooden boats when sealing between a wooden interior bottom and exterior bottom (planks fastened to plywood) For this application a polysulfide would be the best choice?

[pipefitter]

I would think so. Being the planks are definitely going to move a bit in relation to the plywood.I'm not even sure it sounds like a good idea to mix the ply with the reg wood.I never liked the idea of a veneer being trapped somewhere that it can't dry out quickly if water gets to it.

[pcford]

So, from your two replies my research seems to be on track. 3M 5200 should not be used as a bedding material on the bottom of wooden boats when sealing between a wooden interior bottom and exterior bottom (planks fastened to plywood) For this application a polysulfide would be the best choice?

Yes. Or a bedding compound like Dolfinite.

I have made it a habit to put 5200 across the bottom of a transom. (lower transom plank) It is a bad place to have a slow leak because so many members can be affected. You have to destroy the plank to get it off, that is true. But if you are taking off the lower transom plank, you are already in deep do-do.

[PilotArt]

"...Polyurethane is an adhesive (that just happens to be water resistant).

(some) Professionals prefer 5200 for two reasons.
1. Their average customer believes it's the best thing to use for anything.
2. It takes less time/labor to glue things together with 5200 than to properly fasten things together. ..."
Not everything, but to share a couple success stories with 5200:

A friend's 'North Sea Trawler" got driven up on a (Carribean Island, 30 years ago) reef by a hurricane, loosened all her seams. He had her hauled and sealed the seams with many cases of 5200. This 'fix' held her togather long enough to return to the USA for a permeant repair.

My Dolfino Diving Mask, the 'clear poly???' head strap snapped in two, stuck together w/5200 in August 1996 and it still holds.

Yahama 90 TLRP with an Oil Injection Tank on the engine has a Level Sensor/Pickup fitting halfway up the tank that seeped oil, now over nine years since a 5200 coating of wires, pickup tubing and fitting gasket and it has not leaked a drop. :)


"...the idea of a veneer being trapped somewhere that it can't dry out quickly if water gets to it.Water in the internal plies of plywood will not dry out quickly.


"...Why does it seem that professionals prefer polyurethane ..."It is used to attach your windshield to your car, becomes a structural member for roll-over protection and needs a special knife to remove for replacement.

[pipefitter]

I thought windshields were bedded in butyl tape. I remember butyl caulk for glass but in a windshield of a car it would squeeze out in the heat so they had a fibre membrane reinforced butyl tape that was as sticky and tenacious but just didn't squeeze out.

[Texas Boater]

So why do a read about "5200 Bottoms" being such a great thing?


And I know I have read where many professional restorers use the polyurethane as the perfect bottom.

[Figment]

I'll temper my remarks by acknowledging that I'm unreasonably opinionanted on this topic.

The answer to your question, Ivan, is "marketing".
3M has brilliantly marketed their product. Average Joe Boater believes 5200 to be the ultimate boatbuilding product. It's a miracle in a squeezy tube.
Average Joe Boater doesn't know squat about boats.

I suspect that the professional restorers you're reading about are making a buck or two off the labor savings while accommodating the whim of their brainwashed consumers.

agenda? me? pshaw.

[Figment]

I also am under the impression that windshields are set in butyl.
I buy butyl tape from the local auto glass place.

[PilotArt]

3M™ Auto Glass Urethane Windshield Adhesive
(http://products3.3m.com/catalog/us/en001/auto_marine_aero/automotive_aftermarket/node_GS8DWCH715be/root_GST1T4S9TCgv/vroot_GSLPLPKL4Xge/gvel_WWR6DCKBSPgl/theme_us_aad_3_0/command_AbcPageHandler/output_html)
polyurethane ..."
My Bad :( it must have been Urethane not Polyurethane that I was thinking of.

Know nothing of butyl tape... Butyl tape expands 200% to fill... (http://www.manufacturingtalk.com/news/sap/sap101.html)

Looks like just what we need.. :)
_________________________________________

Thank you all for the corrections.

Art

[Gary E]

So why do a read about "5200 Bottoms" being such a great thing?


And I know I have read where many professional restorers use the polyurethane as the perfect bottom.

You read about the sucess of 3M5200 because it works, and it works very well except for a few on this forum who have decided that it's not for them for one reason or another. Then the lemmings who dont actually know anything chime in with the "yeah...whut he said" and pretty soon you have a chorus going.

I kept my Pacemaker in New Jersey, the home of a lot of wood boats many of which used 3M5200 in the underwater seams and many of them do the job one time and it's done for well over 15 yrs with NO leaks. But that was long before this forum and all the Xpert opinions, you know, when we didn't know nuttin.

Ohh and not long ago someone said PL Premium is a substitue at much lower cost. I have no experience with PL.

[pipefitter]

Well hey there GaryE,why don't you tell us how you really feel. :D