Port lights, deck hardware and other
topside applications need to be mounted with a waterproof seal.
Perhaps the most commonly used sealant
these days are the one part polysulphides but butyl is also popular.
Whatever sealing compound is used there are
a few tips and precautions that will make for a better and longer lasting job.
While we don't want the bedding compound to 'glue' the components down
we do want the sealant to 'stick' to them and to whatever they are
So, all surfaces need to be as clean as if one was gluing.
Most new metal fitting will have traces of oil grease and protective
coatings, and that also goes for the bolts and screws.
Clean all the mating surfaces including the heads of the bolts and
When reseating or replacing any components remove all traces of the old
The old stuff might be well stuck on but is it clean and will it be
compatible with the new stuff.
Prepare the mating surfaces as you would if you were going to paint
them, clean, degrease and a quick rub with sandpaper of wire-wool.
Countersink the drill holes in the wood slightly.
small amount of countersinking will not only remove any
rough edges but will also create a small cavity that can be filled with
So even if the rest of the sealant is squeezed out when the
fastener is tightened down there will be a ring of it around the top of
the fastener, just where it is most wanted.
And as you tighten down the sealant will be forced down that
'funnel', into that hole through the wood that is what we
really want sealing
It doesn't need to be a deep countersink.
Another benefit is that the fitting can be tightened right
down straight away without having to wait for the sealant to begin to
fitting require a backing plate in order to
spread the loads from the fastenings across as much of the supporting
structure as posssible.
Fittings that are likely to be heavily loaded will benefit
from having a 'backing plate' on the top of the deck as well as
Stanchions supporting 'guard rails' are particularly prone to
tremendous strain because of the leverage mooring cleats can also at
time be heavily loaded.
Backing plates should be shaped to fit snugly to the underside
of the structure so it has a nice even contact with the surface it
Where it is difficult to judge the shaping, the plate could be
laminated in situ from thinner material.
It is also worth using larger heavier washers than the
standard washers for the particular size of bolt.
Don't bed the inside of the backing plate with sealant, only
seal on the outside.
If there any leaks you don't want the water being trapped
within the wood.
I like to use plenty of bedding compound so that it squeezes
out around the edges and is forced down that countersink.
tape around the fastening before tightening it down
will help with clean up.
Even though sealant in the countersink will seal the fastening
it is still a good idea to turn the fastener around a bit to get the
bedding compound into the threads.
I believe in tightening the fastening down to start
I prefer to have the fitting tightened down onto the wood with
the sealant simply sealing the joint and the edges.
don't want a 'washer' of sealant curing between the fitting
and the wood before a final tightening down.
If I wanted a 'washer' in between I'd use something more
substantial then cured sealant.
However, I do like to give the fitting a final tweak once the
sealant has cured.
Butyl Tape is becoming increasingly popular for bedding deck
fittings and portlights.
Apparently some boat
manufactures are beginning to use it too.
Butyl Sealant has many excellent properties
one is that it
will stick to almost any substrate and is very easy to fit and apply.
It is very flexible and stretchy and while the bond strength
is low is has sufficient adhesion to cling adequately to the fitting
and the deck.
But the main advantage for us boaters is that it doesn't cure,
it retains its same thick consistency for a long time.
And because it never hardens you can keep a roll handy in your
tool box for years when that tube of polysulphide would have gone off.
So it has to be perfect for bedding or re-seating that
occasional small deck component.
The disadvantage is that unlike the polysulphides it doesn't
cure so it won't hold a finish.
And it can be a bit messy if too much is allowed to squeeze
out but it can be cleaned up easily with mineral spirits and a rag.
But because butyl can be softened with mineral spirits it is
best not to use it on fuel fillers or vents.
While butyl tape is stretchy, it is also fairly dense so it
does need a bit of time for it to squeeze out and get into every nook
But as there is no danger of it curing, the tightening process
can be done in stages, allowing it time to spread.
Now I've got to admit that my own experience of using the tape
is recent and limited.
And my main reason for trying it was one of economics, having
something in my tool box that would remain useable for those occasional
The type I have is the black stuff sold in RV stores and while
it appears to be doing the job, apparently it is not as durable as some
Since buying it I have been told that there is a gray colored
variety that is better.
If anyone has more extensive experience of using butyl as a
bedding compound I'd love to hear about and pass on your observations.
For more detailed questions, tips, photos, etc. go to the in the top menu. Forums
Your Comments I've seen butyl used in the past without good results.
Trying to clean it is also a major nuisance.
Depending on the substrate that you're bedding …