Bedding Compound/Sealer

[SkiprJohn]

I will be bedding some plastic through hull fittings in my fiberglass hull. The major compounds are silicon, polyurethane and polysulfide. I've used them all on different projects and know their use.

Most construction books cover the uses of each of these projects but say never to use polysulfide on plastic.

So what in your opinion is the best bedding compound for plastic through hull fittings?

Kind Regards,

John

[Alan Wheeler]

Polyurathane. Keep Silicon away from the things as well. It doesn't adhere well and will leak later on.

[GordMay]

For bedding Through-Hull Fittings, I, like Alan, recommend Polyurethane structural adhesives such as 3M’s #5200/4200, or Sikaflex 291/292/296.
Because Through-Hull Fittings can be assumed to be a “permanent” installation, I wouldn’t worry about the legendary tenacity of Polyurethane adhesives in this application.

~ Cored Hulls also require that you “gouge & backfill” (/w epoxy) the core.
~ All Through-Hulls require substantial Backing Plates to distribute the loading.
~ Polysulfides can attack Plastics, such as Forespar’s Glass Reinforced Nylon “Marelon”.
~ Silicone has no structural adhesive properties, and is only suitable for use as a formable (flexible) gasket, between mechanically fastened fittings. There are two different varieties of silicone - One is based on acetic acid (hence the vinegar odor), and the other is ammonia-based. The latter has little or no smell. The acetic acid version can be corrosive to metal, particularly aluminum. Nothing adheres well to silicone, including itself.
~ Boatlife “LifeCaulk” is a Polysulfide, not suitable for plastics. Their “LifeSeal” is a blend of silicone and polyurethane.

[mike d.]

Hi All!! Mike Here a quick quest. on profiling got e-mail to change or update my profile went been there can't do that (how do i get into my profile to change it thought i read every thing on profile page but ca't seem to find area to change me regards mike

[SkiprJohn]

Aloha Mike,
Did the problem sort itself out? I had the same thing happen the first day I was on but it sorted itself out when I contacted the site host.
Regards, --John--

[SkiprJohn]

Gord May,
I've heard that polysulfide does attack plastic but have a few plastic things bedded in polysulfide already. Should I rebed them or just keep an eye on them? I'm questioning my old transducer installations. I can't remember how I did bed them 15 years ago.
Regards, --John--

[GordMay]

John:
The polysulfide to plastic interface will likely be concealed, so you might not be able to “see” non-catastrophic (to date) damage.
If a solvent attack has occurred, the plastic may have been softened, and you might expect to see some water leaking through the joint.
If a chemical attack has embrittled the plastic, you might not see any visible evidence until some sort of shock suddenly destroys the fitting.
Polysulfide does not attack all “plastics” equally, and some not at all. Additionally, differing polysulfide formulations may use different plasticisers (etc), resulting in differing compatibility properties.
Not knowing the exact compositions of the through-hull and bedding, it remains “safer” to avoid polysulfide-plastic combinations, and to follow the manufacturer’s recommendations for bedding parts.

Polysulfides should NOT to be used on: PVC, ABS, Acrylic (Plexiglass), nor Polycarbonate (Lexan).

As a general rule, Polysufide adhesive sealants are often considered safe for use with: Acetal Resins (Delrin), Polyamides (Nylon, Kevlar), & Glass Reinforced Nylons (Marelon, Minlon)

Notwithstanding, 15 years is a long time in service, and you could be forgiven for supposing that any potential damage would likely already occurred - and that you’ve tested your particular combination of materials in place.

About 20 years is often considered the practical life-cycle for critical structural adhesives.

I’m sorry that I cannot offer more definitive advice on your specific application.

[SkiprJohn]

Gord May,
That is a great response and much appreciated. Very thorough.
With boats I've owned I realize that there is always something to worry about whether at a dock or anchor you might always be concerned about a thruhull whether bronze or plastic. Always something!!
John

[Steve Kidson]

I have had great success and satisfaction from Sika products. In particular, have use Sikaflex 291 for bedding deck and through hull fittings.

Fair winds

Steve

[SkiprJohn]

Have found that 3M 5200 is much cheaper at my local True Value hardware than through West Marine Catalog.
Just a hint.
Regards, --John--