Monday, January 18, 2010

How to Bed, the last

Now all the hardware is up and the deck is repaired and you've got your new fasteners ready to go. Next step is to roughen up the base of the hardware and the corresponding deck surface, using 120 grit sandpaper. You may want to tape off the deck around stanchion bases etc. so you don't sand where you don't want to. Before applying bedding compound I wipe the two surfaces with acetone, to really make sure the surfaces are clean of all wax and oil and sanding dust. Remember, the concept is to allow the compound, which cures to a rubbery consistency, to form a good bond with both the fitting and the deck, so if the fitting shifts a bit under load the bond stays intact.

It is time to apply the bedding compound, but which one? I used 3M 4200, which is recommended for the purpose and performs well. There are other choices, including one or more of the Sikaflex products. I would avoid any oil based bedding compound, such as Dolphinite, unless - perhaps - you are bedding to a wood deck. (Here's a useful discussion.) The oil based products will degrade from the edge in and the result will be failure in five years instead of ten or more. I would also avoid 3M 5200, which is a very powerful, essentially permanent adhesive, not really a bedding compound at all.

Put a 1/16 to 1/8 inch (2 mm) layer of compound on the underside of your stanchion base or other hardware. Do the same to the taped off deck. Press the fitting to the deck and insert the fasteners, placing a little gob of compound under the head of each fastener before you press it down. Compound will of course exude from the work and you need to clean that up before it cures, a good reason for taping.

 Importantly, don't tighten the bolts or screws yet - just press the hardware down firmly but not too firmly. You want to leave a layer of compound under the fitting, a layer perhaps 1/16 inch (2 - 3 mm) or so, and if you press too hard you'll squeeze out all the compound.

Then you will wait until the compound has cured, and only then will you tighten it down. The result will be a fitting that can withstand even a hard direct hose spray, or green water, without leaking a drop.

Suppose your fitting has bolt heads that are exposed to weather, as do many stanchions. You have placed a bit of compound under each bolt head and pressed it down. However, if you let the bolt head rotate while you tighten the nut below deck, the bond under the bolt head will fail and you may have a leak around the fastener. To prevent this have a helper hold the bolt head perfectly in place above deck, using a wrench, while you tighten the bolt from below.

Last year a production boat raced the TransPac right out of the box. Every deck fitting leaked and the race was a misery. The technique I've described here is not well suited to production, because each fitting has to be attended to twice several days apart. I believe many boat builders don't use it, for that reason. Instead, they lay down some compound, then tighten the fitting down hard, leaving too little compound to form a good seal.

There is, actually, a touch of controversy about this technique. In tomorrow's post I'll explain the controversy, I'll describe an alternative, and I'll describe how one can incorporate the alternative into the above-described technique for an even better job.

1 comment:

  1. There's another interesting technique for hardware bedding that is photo-documented by a fellow Maine sailor here:

    As an aside, I have really been enjoying your blog since discovering it. It is smart, curious and informative. I like the tasty blend of modern techniques and traditional knowledge-- all the best the long history of sailing has to provide.

    Warm regards,


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