Sikaflex 295 versus Dow Corning 795

[sailboss]

I need to replace/reseal the fixed overhead acrylic "windows"/portlights/deadlights on my Hunter 36. The posts here reference D/C 795 which is silicone-based. In other places I have seen Sikaflex 295 used. I have also seen folks use silicone sealants on everything from airplanes to boats and never been able to get the silicone residue off in order to use the "proper" polysulfide sealants or even paint over the area.
Which one is better?

 

[centerline]

They are both good....
I have no problem with silicone and its use, but there are others who have had issues with it due to whatever reason they want to claim, and have never taken the time to figure out what the real reason is, so they go with the fool proof stuff everytime... because it seems to work better for them.

The proper prep on any surface is necessary for the best results, no matter what you are applying to it...

 

[alexco38]

Use debond to prepare the surface before applying new. This stuff removes any residue. Sanding / scraping won't.
http://www.debondcorporation.com/

 

[Maine Sail]

I need to replace/reseal the fixed overhead acrylic "windows"/portlights/deadlights on my Hunter 36. The posts here reference D/C 795 which is silicone-based. In other places I have seen Sikaflex 295 used. I have also seen folks use silicone sealants on everything from airplanes to boats and never been able to get the silicone residue off in order to use the "proper" polysulfide sealants or even paint over the area.
Which one is better?

In order to use Sika 295UV the proper Sika 295UV primer, 209D, needs to be used. Dow 795 is a sort of gold standard for structural glazing but Sika 295UV can be used provided the Sika primer is used. Most users are shocked at the price of Sika 209D primer especially when you consider Dow 795 runs about $9.00 per tube..

 

[alexco38]

I found one at Noah boat builder supply for less than $10. It's a one use pencil when you break the glass to use it for 1 hatch at a time. Just a little cheaper.

 

[woodster]

The Dow Corning has a 30 day cure time to reach 100% cured but has served the industry well for a long time ..... When people talk about silicon they usually envision this massive glob of that obnoxious smelly clear stuff that you seal the bath room shower door with not so Dow has very little Oder in it and it bonds very well for its intended purpose I think it has to do with its cemical make up ....it is well suited for plastic/acrylic to fiberglass use

 

[Pat]

We bought a new portlight from D & R after we developed a crack...note I say portlight, not opening port....I say this because
he sent us very easily read instructions that strongly suggested we use DC795....which I promptly ignored. The day comes for the installation and all goes well using the sikaflex...Aboout a month later as the weather is getting cold, I visit the boat and the portlight had shattered into a thousand pieces. I called Rudy to see what I had done wrong and he sent me another instruction sheet and highlighted the Dow Chemical (black) C 795...My wife found the product in KC and I later learned it is what nearly every auto windshield places use because It is flexible and will not crack due to frigid cold or scorching heat in the summer....I still have his instructions if anyone needs them.....the black DC 795 easily contracts and expands depending on the weather without loss of seal or cracking......Patrick

 

[DianaOfBurlington]

Maine Sail-- my Plexiglas deadlights are set in Skiaflex 295, and they leak. Thanks for the head's-up about the primer!

 

[Dan_Y]

The sika primer is impressive in both performance and cost. Hunter (don't know about M-H) used Dow 795 and some others. It's key to rough up the acrylic where the 795 will contact it. Mounting instructions from Hunter say to use 80 grit, but I used 40, just to be sure... I had a bad experience with the 295UV in that it would not cure. Ultimately used the 795. 20 year performance at $9/tube is acceptable...

 

[CloudDiver]

Is there a recommended primer to use for the Dow 795, or just abrade the contact surface with sand paper? I've cut out my new windows from 3/16 smoke acrylic but not gluing them on until I finish the new paint. I will sand the contact surface of the acrylic and the painted cabin, but just wondered if an off the shelf chemical primer is helpful for adhesion of the Dow 795. I'm also figuring out some custom wood 'clamps' to set the windows since I don't want to use thru-bolts or screws.

 

[Dan_Y]

CloudDiver...Look at the table on pg 10 of the DOW manual, second to last entry for acrylic. Just shows solvent wipe to prep surface, no primer. That's consistent with Hunter instructions. Hunter states to sand with 80 grit. I used 40 grit.

 

[Crazy Dave Condon]

Manufacturers use Dow 795 as the standard for adhering windows on sailboats over other products. As one engineer said comparing it to other products, the forumulation/chemical composition is much different designed specifically for adhering surfaces together allowing some flex. It takes time for it to cure. Prep is pretty easy removing the old with a scraper and wet sanding with 80 grit where to be applied. Use alcohol in the clean up only. That engineer said it was like what 5200 is in a sense compared to others and even with that 4200 is different in composition as it is not a permanent glue. I guess it is in the fourmulation of products is the key.

 

[Dan_Y]

The factory 759 in our '91 h30T lasted 21 years on one salon window but other 3 were fine. 795 is warranted 20 years. A perfect case of the product "failing" as soon as the warranty expired! Did a remove and replace and good for another 20 years with no leaks!

 

[Maine Sail]

Please remember, when affixing dead lights to a boat, that there needs to be sufficient film thickness between the acrylic & cabin or hull, This is a critical step that allows for the differing expansion/contraction rates between the fiberglass and acrylic. Shims should be used to ensure adequate cured film thickness. If the sealant is too thin the job will fail and leak.

 

[Dan_Y]

Yes, that's a good/critical point, especially ,if you need to temporarily screw the dead light in place during cure. I used weight on my dead lights because they were flat with no curve. I was able to easily reach DOW tech support (no kidding) and I recall they said I needed either 1/8 or 3/16 thickness of the cured film, on that order anyway for my application. They were careful to point out to use shim material that was compatible with the 795 since it was permanently embedded in the 795.

 

[CloudDiver]

I have been toying with the idea of using 3M VHB tape just to get the initial bond and hold the widow in place which also acts as the shim to prevent excessive squeeze out to the point of having too thin of a film. My window pattern overlaps the window cutout by 1.25 inch, so I will use a narrow strip (3/8") of VHB tape. I'll strip the paper cover from the acrylic only off of the bonding surface and sand prep that surface, wipe with tack cloth and alcohol prep to clean.... then brush on a thin application of thinned 795. I believe this is a better substitute for black paint and prevents seeing the VHB tape through the acrylic after install. After that brushed on coating cures (24 hours or maybe a little more) I'll put on the VBH tape and mount the windows, using a wooden roller to firmly press them. I forgot to mention the mounting surface around the window cut-outs will be sand paper preped and cleaned with alcohol as well. With the windows firmly in place with VHB tape I'll 'caulk' in the 795 around the perimeter of the window, filling the gap but without too much squeeze out.
I think this way I'll have a consistent film thickness, less risk of the windows displacing during install, and not having clamps that could be too much pressure and squeeze out too much sealant. I will also add a similar bead on the inside of the windows. Does this sound reasonable?

 

[Mark Maulden]

About 3 yrs ago, I had all my cabin windows replaced. Notice I said "I had". I usually do my own work, however, this was one project I was willing to shell out a few nickels for. I would have had a total mess. Me and sealants for that big of project just don't get along. Sealants are like a virus. It gets everywhere and even places I swear I don't touch....The yard used Sika and a yr later, they fell out. The yard owner came down and looked at them and promptly stated the primer process was not followed. He redid the job and so far everything is where it should be. The primer and spacing process is critical...

 

[gettinthere]

I hate the mess of trying to get a good caulk fillet...except with 795. That stuff was the easiest caulk to work with. I taped all around both sides of the bead and when the caulk oozed out on application, a wet finger made getting a nice finished look very easy. Then pull up the painter tape and it looked perfect.

 

[Dan_Y]

CD - I used some acrylic from the old dead lights (dark smoked color) and bonded them with a dab of epoxy. the fact they will be covered with a thin layer of 795 seems to make them unnoticeable from the outside. Maybe test first with a scrap piece. 795 is easy to work with, I agree. Smooth and creamy. But once you open it, don't be surprised to find it smeared on spots you were not even close to that day.... Make sure to tape both sides of the gap (edge of dead light and edge of boat) around the opening (re posts 17 and 18), especially on the inside of the boat along the trim. I used the back of a plastic spoon to smooth the 795 on the surfaces and also used it on the gap. Left a nice even surface. You have to clean both sides of the tape pretty quickly, or pull the tape on either side of the gap before it set though!

 

[Crazy Dave Condon]

That day when I responded, my hands were hurting from applying and sanding sheet rock grout. You are right on about the tape as it helps to clean up and I would only use alcohol to help clean with. I use to spit my finger or used a tight glove dipping into the alcohol to smooth out the bead of 795. The spoon is good but dragging too much of the 795 would and will occur. Suggest that dipping it into alcohol frequently will help and also keeping it clean by using a cloth to wipe off. I would always use paper towels but the soft kind is suggested..