{Stus-List} Replacing Windows

John and Maryann Read johnpread at comcast.net
Thu Jun 4 21:43:25 EDT 2009

With the recent interest in replacing windows, we offer the following report
of our research of 4 years ago.  A correction would be that Plexus is not an
epoxy.  As always, there are differing opinions, but we elected to go with a
flexible sealing system.

We replaced our windows using the Sikaflex 295 system with new windows from
Select.  No problems after 4 years and going strong

One sailor's opinion

John & Maryann Read
Legacy III
1982 C&C 34
Noank, CT

-----Original Message-----
From: John & Maryann Read [mailto:johnpread at comcast.net]
Sent: Wednesday, January 26, 2005 9:13 PM
To: candc
Subject: [candc] Replacing Windows

We need to replace the original cabin windows of our 1982 C&C 34.  I want to
share some thoughts and research I have done on replacing main cabin windows
in the hope it may generate some discussion and opinion

Replacing main cabin windows that are glued in without an aluminum frame has
been discussed extensively on these lists and is a common problem for most
boat owners - including C&C.  The seal starts to fail in localized spots and
/ or the acrylic cracks.  There seem to be two main issues:  1)  how do you
remove the old,  and 2) what is the best process and materials to use for
the replacement.

In general, the answer to the first issue is it takes a lot of effort
because the acrylic is epoxied into the cabin side fiberglass.  The window
and epoxy are removed with great effort leaving a very rough surface behind.
The epoxy often literally rips out the gel coat, requiring refilling and
sanding to re establish a smooth surface for the new window.  Some say use a
random orbital sander, others say use a dremel tool.  My feeling is whatever
works is good.  By all accounts, there is no other way to do it.  It is a
messy, time consuming and bear of a job.

The answer to the second issue is more complex.  The conventional wisdom of
the list is to return to the original factory sealant and use the 2 part
Plexus, which is an epoxy.  While it does give an excellent adhesion and
seal, it does have several downsides.
1)  It does not allow any flexibility of the joint which causes problems due
to different expansion coefficients of the acrylic and the fibergalss hull.
The result is the acrylic is prone to cracking
2)  It can be very difficult to apply due to the very short working time and
it requires a special application gun
3)  Care must be taken to ensure adequate thickness around the entire seal
area.  This can be difficult as the acrylic is pressed into place into a
convex frame, effectively squeegying out the center area.
4)  It requires consistent and controlled pressure on the outside of the
acrylic to hold it in place while the Plexus cures - a difficult task
requiring jigs and such
5)  If it ever fails and you need to replace the window again, go back to
the first issue  (Ugh)

Given that technology is always advancing, I felt there had to be a better
way to install the new windows.  I looked primarily in 2 areas - current
manufacturers of boats and sealing of windows in commercial buildings.
Without exception, all advise do not use a Plexus or hard epoxy based
system.  All advised use some combination of:
1)  structural tape which is 2 sided and activated when pressed against to
bond the acrylic to the hull on both sides of the tape, and / or
2)  commercial grade silicone based adhesive / sealer which is formulated to
be UV resistant and has reasonable adhesion properties.sufficient for the

A Google search of the internet shows there are gadzillion different kinds
of structural tape and silicone sealer / adhesive.  The trick is to get the
right formulation.  I approached several OEM boat manufacurers and suppliers
of the tape and silicone and this is what I was advised:

In all cases, ensure the surface of the frame is smooth and well cleaned and
free of any residual sealers or material.  Ensure you are using materials
whose shelf life is still valid.  Sand well both mating surfaces of the
acrylic and frame.  Tape all adjacent surfaces inside and out to minimize
clean up problems.  Ensure temperature and humidity are within spec.  Be
patient - it take 3 weeks for the silicone to fully cure

DOW CORNING - recommends their 795 Silicone Building Sealant.  This is the
stuff they use to seal windows in multi story buildings

SIKAFLEX - recommends a prime coat of Sikaflex 20 then their Sikaflex 295 UV

GE SILICONES - recommends their SilPruf silicone

3M - says they do not make anything that would work

FAIRPORT YACHTS (current manufacturers of Tartan and C&C) - recommends the
Sikflex system

HUNTER YACHTS - uses the Dow Corning 795 system.  They install fastners at
all 4 corners to hold the window in place until the silicone fully cures
(takes 3 weeks).  They then remove the fastners and seal the holes.  So when
all is done, only the silicone is holding the acrylic.

SABRE YACHTS - uses the GE system.  To ensure the window will not pop out,
they also grind down a small circle around the perimeter of the acrylic so
the head of a truss head screw will overlap the edge.  The screw and edge of
acrylic are then covered with Silpruf

CATALINA YACHTS - uses the Dow Corning 795 system.  They also use a series
of jigs with weights to apply consistent pressure to force the acrylic to
conform to the frame curvature.

SELECT PLASTICS - this is a plastics shop in Norwalk, CT which is the
factory authorized repair center for Lewmar and most other brands of
hatches.  They also repair / replace a lot of boat windows.  Some fellow C&C
owners in the area have had good luck with them.  Their new windos do not
leak.  They use a Norton adheisve tape which I have not been able to get
data on and Dow Corning 795 system.

I am leaning toward the tape / 795 system.  The tape will not only provide
adheison, but will also maintain a minimum distance between the mating edges
so the 795 sealer will be effective


John and Maryann Read
Legacy III
1982 C&C 34
Noank, CT

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