[Rhodes22-list] Window replacement update

Tue Jun 2 11:16:06 EDT 2015

For those who are interested in replacing cabin ports with acrylic panels, I thought I would bring you up to date on my highly scientific experiments using the method of attaching windows without using any hardware.

My experiments were intended to demonstrate for me the strength of a joint using Dow Corning silicone # 795 and 3M brand tape VHB 4991 and the paint I intended to use to coat the inside of the acrylic panel.

First, I joined some pieces of Lexan I had using the tape and the sealant and let it sit for 48 hours for the sealant to cure. At  48 hours the sealant is about 90% cured but it continues to cure for 72 hours. I couldn't wait that long. I attached a small piece of lexan (about 1.5" x 3") to a larger piece at a corner. I messed up the sealant with the result that I got some on top of the tape - which seems like a distinct possibility when I do the large panels so I was curious to see what effect that had. By leaving the smaller piece extending beyond the larger piece I had a tab that I could use to peal the pieces apart. I have attached a picture of what I call the "flex test". 

By holding the main piece down and lifting up on the tab I was able to bend the panels far beyond any curvature on the cabin side of the boat without any sign of the joint letting g. Also. the joint had enough flex that the panels could move past each other slightly to allow for a nice even bend. When I bent the joint to the maximum the lexan broke (see photo) before any sign that the joint was weakening. Lexan is very flexible and strong so that seemed to me to be a good sign although I should note that the Lexan was only 3/16" thick. I figure that if I can't break the join even on small pieces without destroying the Lexan, that has to be a very strong and flexible joint.

As an aside, the sealant has a useful life of one year. When I ordered mine it was already 8 months old and expired in March 2015. I called 3M last fall to see if there was any way to extend the life of the material. The technical support person told me to seal it up in plastic and put it in my freezer - which I did. He then told me to test it before using it by laying out a bead and letting it sit for 3 hours. If it skinned over, it was good but if it stayed wet it will never cure - even though it looks fine coming out of the tube. I Had thought that old material would harden in the tube so that was potentially very useful to know. I have read online where others complained that this sealant never cured and I expect they were using material that was too old. In my case it appears the freezing worked as the sealant cured up nicely.

Although the acrylic panels I am using are dark tinted, I wanted to make sure that the grey tape and white sides of the cabin did not show through so I needed to find a suitable paint to coat the inside. I wanted something that would adhere tenaciously to acrylic so that in turn the tape and sealant would adhere to the paint. I also wanted something that would apply by spray to give a perfectly even coat and preferably was fast drying. Articles I read advised using engine exhaust paint. The particular types suggested were not available locally and a ridiculous price to order by mail across the Canada/US border. Also I noticed that application instructions for similar paints available locally specified that it dries easily but does not cure until it is brought up to a high heat. That , of course , is not going to happen with acrylic so I was left wondering how it would work in an uncured state. 

I experimented with a number of paints - especially paints that are intended for plastic. The first time I did this in 1995 I used some sort of enamel that subsequently pealed of even though it was between the Lexan and the cabin side. It was thick, soft and had poor adhesion with Lexan. So I wanted something better than that.  I noticed that the paints I was testing went on thick and took a long time to dry. They also scratched off too easily for my liking. I then tried some barbeque paint that I had. The paint is intended for high heat applications but had a maximum temperature of 600 degrees - which is much lower than the exhaust paint but still much higher than anything the boat will ever face before I am jumping overboard and it does not require heat to cure. It also sprays on beautifully in thin coats, dries to touch in a few minutes - no dust problems - and can be recoated several time within an hour. That meant that I could apply multiple thin and even coats. It gives a satin finish rather than high gloss which I thought might make a better surface for the tape to adhere to. I first painted pieces of Lexan and clear acrylic and let it sit for about 3 months to see if there was any indication of incompatibility between the paint and the plastic. There was no sign of incompatibility that I could see.

I tested the adhesion of the paint by trying to scratch it off soon after application and after several months and by applying a variety of tapes - duct tape and packing tape, house wrap tape etc, - and then peal it back both while the paint was fresh and after the tape had been in place for several months. As the flex test shows I also tried adhesion with the double-sided tape and, as noted, the lexan broke before the seal showed any sign of letting go. I also applied sealant, let it cure , and tried to remove it by pealing it back. The "paint test" is shown in an attached picture. It shows packing tape that had been left in place for several months. The tape was firmly attached but came off cleanly leaving the paint firmly attached to the Lexan. Scratching the paint with a sharp object left a scratch, of course,  but did not remove any of the surrounding paint. So I think this is a good paint for this application.

To prepare the windows, I cut the acrylic panels from a 1/8" plywood template, routed a slight bevel on the outside edges, laid out where the opening in the cabin were, made a template for consistent openings made out of Formica (which works well because it is thin and hard) and then cut the protective paper around what will be the openings using the template and exacto knife. I then removed all the protective covering except where the openings will be and painted the pieces with four thin coats of barbeque paint. I have attached a photo of the painted pieces. 

After the acrylic is attached to the boat I will remove the masking tape on the outside of the acrylic panes, the tape on the cabin sides and remove the protective paper from the outside of the panels and the inside of the open part.  I tested whether I would be able to peel off the inside sections once it had been painted over especially given that it will be difficult to raise an edge once the panels are installed. I found that if I apply a strip of packing tape to the covering paper leaving the end folded over so I can grab it, when I peal it back the tape holds sufficiently to raise the protective paper cleanly avoiding the need to scratch up an edge. Hopefully that works in the real life situation. I would hate to have acrylic panels permanently attached with paper coving the opening that I can't remove. 

So I have chosen all the materials and tested them as best I can for suitability. I have also worked out a temporary clamping system to hold the panels in place while the tape and sealant cure. That might be unnecessary but given the curve of the cabin I don't want to take a chance on that part. I have masked the boat around the edge of the acrylic and sanded the paint on the cabin side so that is flat , smooth but dulled (220 grit). Now all that is left is to attach the actual panels and frankly that is what worries me most. While online instructions suggest applying the tape and sealant to the panel and then apply the panel to the boat, my thinking is that because the cabin side is a compound curve I would be better to apply the tape and sealant to the cabin and then press the panel into place. 

Time matters. I don't have forever to apply the sealant and remove the tape cover before pressing the panel on and worse still, I only get one chance to get the panel in the exact location. There is really no room for error. I have "rehearsed" placing the panel with my wife but that was without any of the tape or sealant. The cabin side has a moulded in edge at both the bottom and the aft end that are very helpful guides but one still need to make sure from the moment of first touch that everything is aligned perfectly so that the rest will wrap into place exactly where it is supposed to be. My hope is that I can rest the bottom edge and aft corner against the little raised edge first and then wrap the piece up into place at the top. 

If anyone has any clever ideas about how I could make the process idiot proof I would be forever grateful.  The idea of having to use a chisel and grinder to remove the misaligned panels has no appeal to me at this juncture. I think I will need a very good shot of Rummy's Mont Gay medicine to calm my nerves first.

Graham 
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