I am writing to update on my portlight replacement project.
The installation went perfectly and the portlight looks great.
Applying a few wood blocks to the inside of the portlight with removable,
double-sided poster tape was a very effective way to affix the portlight in
proper alignment. It slid into position like a puzzle piece, which is good
because you only get one crack at it with the VHB tape.
I also have a very good feeling about the VHB / DC 795 method - though time
will be the ultimate judge. I am comforted by the thought that the tape and DC
795 will allow the portlight to expand and contract with heat and cold at a
different rate that the cabin top without breaking the bond (in theory, anyway).
For those that are thinking of using the same technique, here’s one final tip
and a few comments.
The tip: I had good success using a couple of telescoping hiking poles to apply
pressure to the portlight by bracing them against the lifelines while the VHB
tape set up. I also used a heat gun to warm up the cabin top before applying
the portlight.
I have no leaks in any of the other portlights (yet), so I’m going to wait
until the weather warms up before I start on them. I learned that the initial
bond of the VHB tape is dependent on both the application of continuous
pressure and the ambient temperature. The 3M data sheet does a better job of
explaining this than I will. Once you have a solid bond, it will hold well even
in cold conditions, but you will be doing yourself a favour if you wait for
warmer conditions.
Sent from Windows Mail
From: RPH via CnC-List
Sent: Sunday, February 15, 2015 9:06 AM
To: cnc-list@cnc-list.com
I have been reading the recent posts about windows and I thought that some
might be interested to hear of my experience replacing the portlights on my
1989 30 MKII. Please note that I just started the on-boat part of the project
yesterday and the work is not yet complete.
In late fall I made tracings of the portlights which I transfered to 1/2 inch
ply. The templates were cut using a circular saw with a guide rail system and a
router. I then took my templates to a local plastics manufacturer, and they
made the portlights from 1/4 inch plexiglass (Bronze 2404). The edges of the
portlights are bevelled at 22.5 degrees (if I remember correctly). They turned
out to be very good reproductions of the factory portlights.
To ready myself for the project, I also purchased 36 yards of 3M VHB 4991 Tape
1/2" x 2.3mm, 4 tubes of Dow 795 structural adhesive (I chose white over
black), and a can of DEI 010301 Black High-Temperature Silicone Coating exhaust
paint.
Yesterday, I attended at the boat with a view to replacing only the aft,
starboard portlight (because I didn't want to bite off more than I could chew).
I knew that the factory portlights were glued on with a methacrylate adhesive
and that they would be difficult to remove. I brought my Dremel Multimax (which
is like a Fein Multimaster) and tried using a flexible scraper to get between
the plexi and the cabin top. This was not effective. In the end, I used several
small putty knives which easily removed the portlight.
What the putty knives did not remove was the remnants of the methacrylate
adhesive. I tried several different techniques to remove the adhesive but
ultimately decided that the better course would be to fair the area with 3m
Premium Marine Filler.
After sanding the fairing compound, I held the new portlight in place while my
wife traced the window opening from inside the cabin onto the protective paper
film. Then I used an Exacto knife to cut the paper along the tracing and
removed the paper surrounding the window opening. Next, I spray painted the
inside of the portlight black with the DEI paint so that the tape and Dow 795
will not be visible against the cabin top.
I experimented with small pieces of the VHB tape and was surpised to see that
it did not seem to stick too well to the cabin top even after the surface was
carefully cleaned with alcohol. I decided that the fairing compound should be
left to cure overnight and I will try again today. In any event, I have since
read that the VHB tape needs time to reach maximum adhesion, and that it will
adhere more quickly when the temperature is warmer. I'll bring a heat gun today
to warm up the cabin top before applying the portlight.
Anyway, if all goes well, the plan is to first affix the VHB tape to the inside
of the portlight. Then, with the portlight dry-fitted in place, I will affix a
few wooden blocks to the inside of the portlight (through the cabin top window
opening) using double sided poster tape. My hope is that I can then remove the
protective film from the other side of the VHB tape and then fit the portlight
into place like a "key" into a keyhole. After the tape is set, I will then fill
the gap around the outside with the Dow 795.
That's the plan, anyway.
Robert H.
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