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How can I get a smooth edge on acrylic and drill for screw holes w/o worrying about the sheet cracking? Any special techniques? Thanks.
 

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I've run a fine flat file lengthwise over the edge to round it out.
 

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You can use a belt sander to easily shape / even out the edge, etc.

Have you considered using something like 3M VHB tape, as an adhesive, instaed of drilling / using screws / bolts that will leak later. Also use something like Dow Corning 795 or 791 adhesive sealant to keep the water out.

I redid my deadlights using this method "sexy windows" thread on link below.

Very happy with it. They use the 3M tape to hold big windows in skycrapers, etc.
My old frames were through bolted to the cabin, but the acrlyic was just siliconed to the outside of the frame!!! So, now I have the acrylic sealed to the cabin, with no frame, no holes to leak. I tapered the black Dow 795 sealant along the edges, to give it about a 45 degree taper.


Was not able to upload pics here!!

Dscn0808 - Cruisers & Sailing Photo Gallery

new "sexy" windows - Cruisers & Sailing Forums

HOW TO HAVE SEXY WINDOWS!? - Cruisers & Sailing Forums
 

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Wet sanding after cutting with a fine tooth blade on a table saw works, but is a bit labour intensive. There are special drill bits available for drilling plastics, they are worthwhile picking up, also be sure to drill the hole oversize for the fasteners so that you don't stress the hole during expanion/contraction.

We have also had good luck with Dow Corning 795.
 

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Have you considered using something like 3M VHB tape, as an adhesive, instaed of drilling / using screws / bolts that will leak later. Also use something like Dow Corning 795 or 791 adhesive sealant to keep the water out.
Dow 795 and Dow 791 are both silicon based sealents, so if you are planning to seal the acrylic against the fiberglass you may want to consider a different adhesive. As has been discussed here, silicon is bad for fiberglass.
 

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Discussion Starter #7
How fine should the blade be? I only have a skill saw with a blade I used to put my flooring in. It's fine toothed........for wood.
 

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Im in the process of the same job, and I am considering (have not decided yet) using 3M 560 or 540. 3M says:

3M™ Scotch-Seal™ Polyurethane Sealant 540 and 3M™ Scotch-Seal™ Polyurethane Adhesive Sealant 560 are one component, moisture curing products which form permanent elastic bonds. They bond to a wide variety of materials including plastics, FRP, SMC, aluminum, steel, coated metal, and wood.
Here is all the data:

http://multimedia.3m.com/mws/mediawebserver?66666UuZjcFSLXTtnxf25XF6EVuQEcuZgVs6EVs6E666666--

The only catch is I only see 540 on sale on their web site, can't find 560. 3M says that 560 has a tensile strength of 580psi and an elongation before break of > 300%. For windows that might be a good strength, although if you want less than the 540 has 250psi and > 800%.

I can't say I know just how strong a dead light bond should be, hopefully someone who knows will chime in ;)
 

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One problem is that the saw will MELT the plastic as you cut SO you need to keep it a bit wet and work slow

Then you can use a sanding block to clean up the edges
 

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Discussion Starter #11
Would 4200 work as a sealant? Or does it not strech enough?
 

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Discussion Starter #12
One problem is that the saw will MELT the plastic as you cut SO you need to keep it a bit wet and work slow

Then you can use a sanding block to clean up the edges
Thanks for the tip tommays. :)
 

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Would 4200 work as a sealant? Or does it not strech enough?
I'm actually on hold with 3M right now trying to get that very answer ;)

3M's web site seems says 4200 is "medium strength for disassembly." I a trying to get the tensile strength and elasticity.
 

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jarcher - I have heard lots of people advise against old style silicon, for various reasons( such as temporary adhesion only, the niusance or removing it from gelcoat, in order to repaint, etc), but I have never read that it would harm the fiberglass / gelcoat. The "sexy windows" thread, in it's original link to Bob Norson's article, suggests the dow 791. I found Dow 795 locally, and was extremely happy with it. Others have suggested Sika 295"??, which sounds ver ygood as well.

ZZ- I would not use 4200. You want something that is able to stretch much more, as the acrylic has a high elongation rate with temoerature changes.
 

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jarcher - I believe I used VHB 5952. It was black for sure, and I remember looking for one of the thicker tapes, as I put a good bead of Dow 759 a cm away, that went between the acrylic and the gelcoat (providing more adhesion, and a watertight seal), and spread out around the edges as well.

I wanted thick VHB tape, as the thinner it would be, the thinner the bead of Dow 795 would have to be. And you want it thicker, so it will elongate more.

Here's a link to the tape comparisons, and 3m store:

http://multimedia.3m.com/mws/mediawebserver?66666UuZjcFSLXTtnxMtLXs6EVuQEcuZgVs6EVs6E666666--

Shop 3M: 3M VHB Heavy Duty Mounting Tape 5952 Black, 3/4 in x 15 yd 45 mil
 

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... but I have never read that it would harm the fiberglass / gelcoat.
I misspoke (mistyped). The issue with silicon on gel is that (according to the books and threads I have read) gel does get porous, and the silicon gets in there and is very hard to get out.

Once its there, nothing else, including more silicon, will stick. So if you ever need to rebed or replace your dead light, you may have trouble sealing it again.

Of course I never argue with results, and if it works well for people good deal.

3M just emailed me all the relevant data sheets and I am waiting for them to arrive. The customer support person suggested 4000UV is the best choice for a dead light but she said that you still need mechanical fasteners, which apparently cause other problems.

I'll post the info when I receive it, which should be any second...
 

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Though DC 795 is silicon based, it is not of the "vinegar smelling" variety, which, I believe, is the "problem" silicon to use on boats in general.

We have done several boats with it, and years later it still stands up OK and has not leaked.
 

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there have been many threads about the difference between fastening and sealing (ie if you have mechanical fasteners - screws / bolts, then you can get away more with a sealant, from a gun or something like butyl tape - Mainsail / Halekai has lots of nice pics and comments in his posts )

I choose, as many are doing now, to use the VHB tape, as the mechanical fastener, instead of screws / bolts that are prone to leak. I added the Dow 795, which is both a strong adhesive (mechanical fastener / glazing silicon) as well as a "stretchable" sealer, that will stay adhered to both surfaces as they elongated or shrink at different rates.

I can tell you that I put in one portlight / deadlight by mistake, and had to remove it seconds later. The Dow 795 was obviously not cured yet, so the only adhesion was the VHB tape, which I had not even pressed on hard. It was all I could do to use my whole body weight to push it out. Now that they have cured, I doubt they could be pushed out - maybe after many hard kicks!!
 

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Though DC 795 is silicon based, it is not of the "vinegar smelling" variety, which, I believe, is the "problem" silicon to use on boats in general.

We have done several boats with it, and years later it still stands up OK and has not leaked.
Here is the data sheet for Dow 795:

http://www.meiglobalsolutions.com/images/dowcorning795.pdf

I should point out that Tony at Select Plastics highly recommends this stuff for adhering dead lights to gel coat, and recommends against using any mechanical fasteners.

If I read the data sheet correctly, the elasticity is 50% and the strength at 50% elasticity is 50psi. If that is all that is required to secure a dead light, then there are probably many other options in adhesives.

But I may have read it wrong, I didn't get much sleep last night and I am starting to fade ;)

Still no email from 3M :(
 

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In my case, most of the adhesion is provided by the VHB tape. As the two surfaces expand / contact at different rates, the Dow 795 doesn't have to keep the windows on the boat, it just has to stay adhered to both surfaces, so it remains sealed. I know they expand alot, but in real life, I doubt if the 795 is being expanded to the full 50%, unless you have very long or wide deadlights.
 
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