Selecting a Dow Sealant

[Neil McDonald]

Some time ago this forum persuaded me that 795 was the choice Dow sealant for marine glazing. So I have just used 795 to install my replacement cast acrylic hatch lens - which I cut from a blank. I was subsequently a little surprised to see that the curing time is quoted by Dow as 168-336 hours.........that's a lot of time in the middle of the boating season!

Dow calls 795 a "Silicon Building Sealant". Previously I had been using Dow 995 - which they describe as a "Silicon Structural Sealant". Apart from the fact that work time for the 995 is a little shorter than for 795 (10-20 mins vs 20-30 mins) the main difference is that that the tensile strength of 995 is 350 psi vs 45psi for the 795. Both materials are capable of plus or minus 50% deformation.

As my hatch lens has no mechanical retainer system and relies on the sealant to hold it in place, I will choose the 995 next time around. Perhaps I should have done this detailed research before I tackled this job!

 

[Rick D]

My Port Replacement Guy

...used 795 and only recommended one week of curing before use. He's a professional, so I trust his advice. BTW, 795 is what they used to glaze in our office building windows.

 

[KD3PC]

hunter specs 795, as do many professional shops for port replacement as well as MANY pro glass installers in all weather situations. Even DOW spec'd the 795 for me.

Your boat, you decide, but I will go with what the "experts" call for.

It was good enough to last since 1991 on all my ports, except two....if I can get 15-17 years out of a sealant on two dissimilar materials in an all weather environment, that will still come off when you want to repair/replace the port, that works for me.

Sorry the list let you down, let us all know how the 995 works for you.

 

[Neil McDonald]

Not a Problem at This Time

Sorry the list let you down, let us all know how the 995 works for you.

I don't think that I was let down! I used the 795 for this job and only time will tell how it works. The rubber gasket underneath is pushing the lens upwards and only the sealant (plus a ton of temporary weight for the next week!) will be holding it in. The only reason that I posted was that the 995 specification appears to provide better performance..... although the working time is shorter.

 

[John Nantz]

995 spec'ed by Northwest Dow factory Tech Rep

Hi Neil! I was the first one to recommend the use of 995 on this board and the reason was due to the recommendation of the Dow factory Tech Rep for the Northwest.

The problem I had was with sealing the Lexan skylights and they would wind up leaking after only a fairly short time. This started out with me talking to Dow back east and finally ended up with me talking to the Tech Rep. This is their guy who deals with "issues" in the field, some of which become legal.

Believe it or not, he actually came out to Poulsbo, which is somewhat off the beaten path, and looked at my boat and the problems I was having. It was after this that he suggested I use 995. I knew that Hunter Marine used 795 and that is why I went directly to Dow in the first place.

Keep in mind that boat builders like Hunter don't necessarily use the best, but maybe the best for the money. They use TONS of sealant - just look around on you boat. This is what they use to hold down the floor boards (okay, cabin sole), put in many of the cabinetry joints, etc., and etc. They buy in quantity and don't like variety. Hence, 795 everywhere. Also note that 795 is slightly cheaper than 995 (but enough to make a difference - a penny here and a penny there...).

The Tech Rep told me that 995 is what was used to hold the glazing in skyscrapers - not trim, not screws, but sealant. This was many years ago now, back in the '90s, and I've only had to do a couple repairs since then. No leaks!

Also, the Tech Rep said there is a particular way to use the sealant. First off, it is NOT a "bedding compound," consequently it should be used differently that that. It is an ELASTOMERIC compound. Read "rubber band." It is NOT to be spread on the recess and then the "glazing" (read: Lexan skylight) placed ON it, it is to be put on the EDGE of the item and then SHAPED (very important!) so it can act like a rubber band that is glued at each end (edge, in this case). I used a "backer rod" to get the lower shape and my finger to get the upper shape on the skylight joint.

For a hatch window I don't know if this would be the best thing or not. I think this is more of a "mechanical" seal than an elastomeric seal, but who knows, maybe it would do fine there. Also, I don't know how it would react with the hatch plastic. With the Lexan it does fine.

The 795 is an older design and when I got the 995 it was quite new and at that time one had to be a Dow licensed user in order to buy and use it. I was an exception, probably because the guy thought there was no way someone would be hurt by falling glass if I messed up.

So, that's the story.

Maybe in the archives there might be a diagram I made and posted of the cross-section for proper installation. There was an architect who read it and wrote a nice reply about my installation description and thanked me because he now understood how it worked and why his skylights leaked. Architects learn about these things so he immediately made the connection. By the way, I'm no architect but I've worked with some. Okay, so now you can feel sorry for me if you want! (Duck!!)

Hope this helps.

P.S. Use BLACK sealant if you can because it has better UV resistance. Also good advice from the Dow Tech Rep.

 

[John Nantz]

Tensile Strength difference

One more thing to look at is the tensile strength that you mentioned, and it's a good thing that you mentioned it. That is a HUGE difference. No contest.

That speaks volumes in the case of the skylight installation. For the hatch - I don't know because the metal frame goes all around.

 

[Neil McDonald]

One more thing to look at is the tensile strength that you mentioned, and it's a good thing that you mentioned it. That is a HUGE difference. No contest.

That speaks volumes in the case of the skylight installation. For the hatch - I don't know because the metal frame goes all around.

I feel better now. Having received several well meaning responses along the general lines of "795 is what everyone uses, it must be the best" I was starting to doubt my ability to analyze technical specifications! Since I make my living as a consulting engineer (not in this field though).... this feeling of doubt was a little disconcerting.

I had previously used 995 to re-install skylights and it worked well. I probably got the idea from you!! Glazing a Beckson hatch is very little different from the skylight application. This is because the lip of the hatch does not go over the lens to hold it in place as you might expect. Looking straight down at the hatch, without any sealant in place, there is about a 3/16' gap between the edge of the lens and the edge of the frame and there is absolutely nothing but the sealant to stop it from flopping around. Hence the hatch lens is effectively "edge glued" in place in just the same way as for a skylight. The only difference is that the hatch has a removable rubber strip located behind the glazing to seal the hatch to the deck when closed. This strip also serves as the " backer rod" when applying sealant which prevents the sealant from migrating too far.

Dow has lots of "how to install" technical material online, but there is so much stuff there that it is hard to find exactly what you want!

The attached picture shows the new lens sitting in the frame just before adding sealant. Everything is masked with tape. Note the temporary spacers to keep the lens in place when adding sealant and the hole for the fastener. All edges are chamfered to reduce the risk of stress fractures. A rag soaked with water does a wonderful job smoothing the sealant in place.

A new hatch lens for around C$65 in materials is a good deal. Just don't ask about my time.

 

[John Nantz]

Neil - I'm impressed

A while back I was going to replace the "glass" in the forward hatch because the old one was really craized and bought the new plastic lens but never did it. Still have it somewhere in the garage. Gave it a lot of thought and said to heck with it. Okay, so maybe I didn't use those exact same words.

Wound up buying a new hatch instead.