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By Bob Norson When we were rebuilding “WhiteBird” some years ago, I was told that new boats were being made with windows/ports held in with two sided tape and silicone. I was replacing the old glass with polycarbonate but there was no way I was going to use wimpy shit like two sided tape to hold her together. In fact, there wasn't even enough screws left from the aluminium bezels to satisfy so I went to work with drill and tap and bought boxes of 5 mil screws…. about 500 all up! When finished it was a work of art and admired by all, zillions of shiny screw heads holding the dark tinted plastic against the neatly trimmed Black Sika Flex on sparkling white painted sole and combing. The only thing worse would have been a bunch of varnished teak!! I got away with it for a long time because I am always careful on boat work and fussy of finish but something I've found about working on boats is this… PERFECTION IS ONLY 90% EFFECTIVE! Besides Sika Flex not being up to the challenge (not the best anyway) of sealing plastic with the expansion tendency of “Lexan” or polycarbonate all those screws were an invitation to trouble. Some were bound to leak over time and they did. On a plastic boat that means you have a tiny leak. On a steel boat a tiny leak means you have a big problem eventually. As the water penetrates the sealant around the screw and begins to corrode the steel behind the plastic, the expanding rust scale wrecks the sealant against the plastic itself and … well you get the vicious circle idea. I had four things to do; first was to get the plastic. In Australia, Austin Glass in Townsville is the place. They cut to size, ship anywhere and the owner, Dan Hughes is a boaty and knows what you need. second was to repair the paint system, which on a steel boat deserves a couple pages of it's own. (See TCP # 27 for the full article in free downloads section) Third was to find the right sealant which wasn't as easy as you might think but that is covered further along in this article. Lastly to mount the windows without screws and still make it look sexy….. the dreaded two sided tape The window tape needs a relatively even surface to hold the plastic on. If there is more than gentle curves involved use Wattyl brand fairing compound between the primer and top coats somewhere to even things out. It's a two part putty like compound. Unless you let it go too long the stuff sands fast and leaves a great surface. Besides the mechanical issue it will look better when done. The fairing doesn’t need to extend beyond the area where the tape will be. If the sealant is a few mill thicker in one spot than another, no big deal. The sealant hides the difference well. All the paints and the fairing compound mentioned are products I have experience with and believe are the best for the job. All of these products are available from Whitsunday Ocean Services in Airlie Beach, phone (07) 4948 1366. Even if you are doing your thing in Tasmania it may still be cheaper to order from them and pay shipping than buying local. I have observed great disparities in price on these materials. So.. we all have an even surface to work from now, with a firm substrate of paint or other durable, UV resistant surface. READY STEADY GO!! Cut your plastic to size leaving a 40mm (1 ½”) overlap (some say less but I work on the “safe” side) for your tape and sealant. I used a metal cutting blade on a jig saw running at moderate speed with good result. A less than perfect cut can be straightened out with a belt sander or file or whatever but don't polish the edge as you want silicone to stick to it later. Leave any plastic film in place on the plastic and note that Lexan has an “in and out” side. After cutting out you need to paint the area that will be the overlap facing in. Either use the plastic film that came with it or mask off with tape. I used a medical scalpel for shaping the film for masking. Xacto brand knives should work as well. You can get primers for plastic then coat with poly-U 400 but I found automotive exhaust paint to be very tough and grippy on the shiny plastic. (This was an insider trick I got from a lexan disributor!) Painting on the underside edges reduces UV that could get to the tape to degrade it but more importantly, hides all the stuff so it doesn't look shithouse. (Lexan brand polycarb is claimed to be UV proof anyway.) Black was the colour appropriate for my application. Next…. The very important task of masking off the sealant can be done now or later. I chose early because I wanted to apply the window with some sealant goohed on the boat already. Some say that extra sealant isn't needed, they squirt it in after the window is fastened with the tape but because I was worried about lack of sealant and working time later, I put masking tape on the boat then held up the window and used a divider to mark off a line about 5mm over the size of the window. Once marked I carefully (so as not to penetrate the paint surface) cut a line with my scalpel and lifted off the excess tape. Now strip off all the plastic film or masking tape you applied for the painting of the window and apply a mask to the edge of the outside of the new window to protect it from the sealant. I just laid it down quick and sloppy, then ran my blade around the edge and viola.. done.
Next… 3M brand tape is known to be very good. It comes in one inch and ½ inch widths. I used two strips of ½ inch to make the recommended 1 inch total. The best holding 3M stuff has a red cover. There are others but I used the 3M that I found in Mackay. Apply the tape to the boat toward the inside of the 40mm overlap area. Next……Now strip off the cover on the two-sided tape and optionally, squirt on a thin line of silicone under where the edge of the window will be but not to close to the exposed tape surface. When the window is applied you don't want the silicone to invade the tape. Raise up your prepared window and carefully…..oh so carefully put the window in place. You only have one shot at this. If it's wrong you are screwed, blued, stuffed and tattooed. Don't think you can move and slide, you can't. It's either right or get out the hammer and chisel. Assuming its right, use your palm to push hard to really set the tape to plastic. Now that you are considering that stress, lets talk silicone! The right stuff is hard to find. You need a silicone of the correct colour (black in my case) with a neutral cure. If using Perspex you may get away with only a 20% + or- joint movement capability. With lexan or polycarbonate you need more because it's rate of expansion is greater. I found the right stuff at “Lincoln Sentry” in Mackay. It is Dow Corning brand, silicone # 791 with a whopping movement rate of 50%!. Schionning Marine also carries the right stuff. 3M also has a sealant intended for use with Lexan, consult your local dealer. Next…. If you haven't already, mask off the outside now. (about 5-6mm from the edge of the plastic on a flat surface or to the edge of a rebated surface) Squirt a line of the silicone all the way around the outside edge. With a rubber glove covered finger, work the silicone into and under the edge of the plastic and finally smooth off the whole way round. Don't leave any holes or rough spots. Now carefully pull off the tape. Don't wait till it starts to set. Get it now. Rubber gloves are cheap at the supermarket so use plenty of em. Keep a bottle of methylated spirits and paper towels handy to wipe up the stray accidents. DONE!I thanks to: |
