When I replaced my portlights this spring I had planned to follow Don Casey's article but ended up not using the VHB.
I have had good experience with VHB on a residential installation of steel panels as a surround for a gas fireplace. The VHB is bonding mild steel to painted drywall in this case and performs very well considering the temperature variation. So I like it and what it is capable of. When it came to the portlights, I had read that the expansion of the acrylic relative to the fibreglass is what causes the bond to fail, or could cause crazing/cracks in the acrylic. To get enough movement/flex out of the adhesive (or tape) it should be 3/16" to 1/4" thick between the fibreglass and acrylic. My portlights are approximately 60"x8" btw. I couldn't source the thicker VHB in Toronto, aside from a couple other concerns. The Casey article mentions backpainting the acrylic to hide the typically grey colour VHB. I became worried that the paint used then becomes the bonding surface which I can't really prove sticks as well to the acrylic. There was also a worry that the chemicals in the paint could etch and/or cause crazing in the acrylic -even the data sheet for Krylon Fusion had things that attack acrylic in it. I decided it might be best not to risk using the paint and just double up the thinner VHB that I could source locally. I then got to worrying that the VHB might not be UV stable behind the acrylic... and I didn't want to repeat this task for a while. Finally, I picked up a case of Dow 795, since that was all I could find around here for quantity. I had read that you could install screws into the window recess to act as spacers for the 795, giving enough gap to allow the silicone to flex with the acrylic. I was concerned that the screws would be visible which didn't seem so great. I thought about the spacers that glass installers use for shower surrounds and mirrors and then thought about making spacers with the 795 itself. I spoke with a tech rep at Dow and he confirmed that unlike typical run of the mill silicone, 795 would in fact stick to itself. So I gooped lines of 795 on some scrap plastic, waited a week for it to cure, and cut it into little tiny bricks about 3/16"x3/16"x1". I did a tiny blob of 795 behind these in the window recess with the spacers every 8" or so and then applied lots of 795 to bond the acrylic. I also sanded the bonding surface of the acrylic with 220 and wiped with isopropyl alcohol to clean. In retrospect, it would have been good to let the spacers cure before doing the acrylic since some of them shifted around during installation. To hold the acrylic in place, I made an MDF piece slightly smaller than the acrylic with plywood blocking screwed into it every 10". I clamped a 2x4 to the stanchions and with more plywood blocks, screwed everything together to apply the curvature to the acrylic and provide even pressure across the whole thing. I let the 795 cure for 10 days or so since it is very slow. My other worry with the acrylic was that an edge flaw could cause crazing, so I ended up band sawing the shape within 1/8" of the original, trim routing it flush with the original and then sanding from 80 up to 2000 grit to polish the edge. I also did a 1/8" chamfer on the outside since I thought it would look nice. They were installed this spring and have been great this summer. I think the real test will be surviving a Canadian winter and seeing how things look next fall. Derek McLeod 1983 29-2, Aileron Toronto > On Oct 21, 2015, at 10:13 PM, Gary Russell <captnga...@gmail.com> wrote: > > Fred. > Based on Don Casey's article, the 3M VHB tape is for adhesion and the > 795 DOW Corning silicone sealant is just for sealing. No? > Gary > > ~~~~~~~_/)~~~~~~ > > >> On Wed, Oct 21, 2015 at 8:55 PM, Frederick G Street via CnC-List >> <cnc-list@cnc-list.com> wrote: >> Based on the data, it sounds like VHB with LifeSeal would be a great >> combination (four times stronger than with Dow 795). Has anyone tried that >> yet? Or am I going to be the first guinea pig? :^) >> >> Fred Street -- Minneapolis >> S/V Oceanis (1979 C&C Landfall 38) -- on the hard in Bayfield, WI :^( >> >>> On Oct 21, 2015, at 7:18 PM, Dennis C. via CnC-List <cnc-list@cnc-list.com> >>> wrote: >>> >>> LifeSeal is my "go to" sealant. Haven't used 4200 (polyurethane) or >>> silicone in months. >>> >>> Dennis C. >> >> >> _______________________________________________ >> >> Email address: >> CnC-List@cnc-list.com >> To change your list preferences, including unsubscribing -- go to the bottom >> of page at: >> http://cnc-list.com/mailman/listinfo/cnc-list_cnc-list.com >> >> >
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