|Topic Review (Newest First)|
|04-23-2009 12:06 PM|
|tigerregis||Note that Maine Sail spells silicone correctly. There is a big difference between silicon and silicone.|
|04-23-2009 11:14 AM|
Originally Posted by jarcher View Post
Please DO NOT confuse Dow 795 with your basic Silicones. Dow 795 is a structural silicone and it is what is used by most boat builders to bond dead lights to fiberglass.
This is NOT an application for bathtub Silicone or general purpose "marine" Silicone. Dow 795 is tough stuff with millions & millions of blue water miles behind it over many years of service life on thousands of vessels...
Before using ANY sealant the surface MUST be properly cleaned and this will require sanding to remove ALL previous Silicone contamination. Even Dow 795 will not stick to a surface previously contaminated with Silicone.
Polysulfides and many polyruethanes should not be used on certain plastics as they can leach the plasticizers out of them, over time, and leave them prematurely brittle and prone to failure.
|04-23-2009 10:33 AM|
|Northeaster||jarcher - not all silicons are the same. The normal, cheap silicon would not be good for this application. The recommended products are the Dow 795, Sika 295, and maybe one other that Mainesail mentioned. Yo ucanneot go worong using one of these!!! I would also advise using the 3M VHB tape as the primary adhesive, as mentioned earlier. YOu can have as much tape and dow/sika as you want. (ie you can overlap your deadlight 2" if you want, which would leave enough room for 3/4" or 1" of VHB tape, and then a gap, then a nice bead of Dow or sika that extends out to the corner, to be faired against the cabinsides later.|
|04-23-2009 10:04 AM|
Originally Posted by Maine Sail View Post
So my question is, why the big disagreement on silicon, at least for this purpose. SD has said over and over (and he cites good reasons) that silicon has very few uses on a boat, those perhaps being limited to the tips of cotter pins and the underside of dishes.
Tony and Mainsail, at least for this purpose (bonding acrylic dead lights to gel coat) say Dow 795 (silicon) is the way to go. And, the Dow data sheet does say that 795 is suitable for bonding glass to buildings in high wind applications, but buildings are not made from gel coat.
Another poster here commented that perhaps the issue with silicon is limited to that which has the vinager smell.
And then there are the products that have some silicon in them.
Is this just a difference of opinion, or is using silicon to bond the windows one of those exceptions where it is appropriate? If the later, what happens when it is time to rebed or reseal them?
I don't mean to start a war or be a pain, but I am in the process of replacing dead lights. If I don't use fasteners of any type, and the dead light pops off while I'm on my ear, the boat could easily sink as water rushes through the 45 inch opening that is left behind. Hence my concern not just for the initial installation, but the quality of the bond should it need to be done again. So if the silicon makes it difficult to reseal later, I would prefer to use something else, even if just for my own mental calmness. If there is a reason the silicon is not an issue, then so much the better.
|04-23-2009 07:11 AM|
Great thread guys. When I built my windows, I cut them out with a router. Worked great, no worries about breakage during the cut, and was an easy edge to clean up. You can even use an angled bit to bevel the edge as you cut them out. I through bolted my windows, mostly because I had no idea how else to attach them. Drilling can be a problem though. A regular drill bit will try to screw itself into the acrylic. The result will be cracked acrylic or the backside of the hole blown out. There are 2 solutions, the very best way is to buy an acrylic drill bit, the other is to make one. To make one, simply take a new drill bit and dull the crap out of it in a grinder. You have to reduce the "angle of attack" on the leading edge so it can't screw itself into the acrylic.
I used Boatlife's LifeSeal silicone on mine, and made sure I didn't squeeze all of the silicone out. I soon found out that the surface of the window where the silicone goes must be roughed up with sandpaper or the silicone won't stick for very long. On the second go-round I used Permatex Ultra Black sealant from the auto parts store. Worked like a charm and looked better than the clear I used the first time.
|04-23-2009 05:48 AM|
I cut mine out with a band saw, then sanded the edges.
|04-23-2009 05:47 AM|
Originally Posted by Maine Sail View Post
|04-23-2009 02:31 AM|
|timebandit||Cut it with a tile saw and drill with a step bit.|
|04-22-2009 07:31 PM|
Guy you don't need to take my word for this. Here are some quotes from Tony D'andrea who is the Acrylic & Polycarb guru in the marine industry. He is the Nigel Calder of portlights and hatches and owns the largest hatch warranty & repair facility in the world..
Tony on Acrylic vs. Polycarbonate:
"Consider the Following:
Additional considerations may include polycarbonate with UV and scratch resistant coatings. While these products are heavily promoted by several manufacturers and carry 5, 10 even 15 year warranties the following information has been reported in "real life"applications:
Both Acrylic and Polycarbonate have specific uses and installation requirements.
Cast Acrylic (of a specific thickness) is in accordance with CE and ABYC guidelines, and installed on virtually all of the big blue water sail boats produced on both sides of the pond. Polycarbonate is commonly used as a replacement due to its ease of fabrication and incredible initial strength. The USCG and USN require Polycarbonate on their vessels but they also have a PM cycle of 36 to 42 months for change out. My Tax dollars at work...
Due to its ductility Polycarbonate it is more challenging to install. I have seen Sika Flex 295UV with primer and Dow 795 both mentioned. I use and recommend both. Dont go over 4 ft continuous length with a fixed portlight. Remember the coefficiant of thermal expansion for Acrylic and Polycarbonate is in the neiborhood of .000039 per inch per degree F. That means an 8ft plastic port will expand and contract up to 1/2 of an inch from the coldest day in Feb to the hottest day in summer. WOW!! Compartmentalize the job. It will be easier to install and less prone to leaks.
Never ever bolt a plastic portlight in place. Screws are fine to hold a lens till the adhesive cures. Take them out asap and fill the holes with the afformentioned products. Both of these products are rated at 700 + percent elongation before tear. Strong flexible and UV resistant.
Been to a boat show lately? Seen any screws? Glass is good so long as your boat does not twist or torque. Show me a fiberglass boat that does not twist and I will show you a cocktail barge tied to the dock."
Tony on Sealants:
"I hear the question as to which sealant to use when bonding Acrylic, or Polycarbonate to aluminum, stainless or FRP over and over and over.....
Well here goes... The only three adhesives I would consider using are Sika Flex 295 UV with the primer, GE SG-4000, and Dow 795. Using the correct adhesive is only 1/2 the battle. Do not apply the sealants below 50 degrees F. The temperature must maintain at least 50F during the entire 21 day cure cycle. Cut this corner and your finished before you start. Preparation of the bond area is also very/ very important. DO NOT TOUCH THE BOND AREA WITH YOUR BARE HANDS! Contamination from the dirt and skin oils will make a solid cure impossible. You may clean the FRP and metal with acetone to prep the area but if you touch the Lexan or Plexi with harsh solvents you will ruin the portlight. A 50/50 mix of isopropal alcohol and distilled water will work well to clean the plastic if needed. Remember that clean enough is not clean enough."
Those are not my words they are the words of one of the most experienced marine plastics guys on the planet..
So this means DO NOT screw dead lights into place and DO NOT use standard sealants like 4200, 5200 or any of the others. If there were a better sealant for cast acrylic Tony would mention it. Use Dow 795, GE SSG 4000 or Sikaflex 295 UV with the special primer. Oh and acrylic not polycrabonate but the OP chose the right material anyway..
To answer the OP: In order to get good clean cuts make 1/4 plywood templates and then have the cast acrylic CNC cut by a plastics fabricator using your templates. You made the right choice with acrylic but please don't try and through bolt them or they can most definitely split..
|04-22-2009 07:17 PM|
Have you thought about using a wood burner or soldering iron to make fastener holes in the acrylic? I used this trick on a thin sheet that I used to hold my boat's registration numbers, and it worked like a charm.
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