# Bonding aluminum to polycarbonate: primer?



## cghubbell (May 8, 2009)

I need to bond new sheets of polycarbonate (Makrolon) into the old aluminum frames from my boat. I'm planning to use Dow 795 structural silicone. My ports have no mechanical fastening. That is, the Dow 795 will need to hold the sheet in place. I have seen mention in some places about using a primer, but I can't seem to find any guidance on when you do or do not need it.

Does anyone know whether the Dow 795 will sufficiently bond the aluminum and polycarbonate without a primer? And, whether or not the bonding surfaces need to be lightly abraded to aid in the bond?

_Disclaimer: I am aware of the lexan vs. polycarbonate conversation. I need to use what I have, and I'm fine with replacing it in a few years if a problem develops. Just need to make the best of on-hand materials._

Thanks in advance,
Chris


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## cghubbell (May 8, 2009)

To clarify, the aluminum frames are mechanically fastened to the cabin, but the makrolon "floats" in the aluminum frame, held in place only by the silicone.


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## sailingfool (Apr 17, 2000)

These guys have some instructions
Which Sealants To Use

If you remind me who made the hatch, I may be able to find the link/address for a US distributor of replacement lens and related sealants. Back when I rebedded my CS 36T hatches with a silicone kit with primer I bought from the distributor it all worked great.


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## cghubbell (May 8, 2009)

sailingfool said:


> These guys have some instructions
> Which Sealants To Use


Yes, this is where I picked up the cue to use Dow 795. It doesn't answer the question about primers though...



sailingfool said:


> If you remind me who made the hatch, I may be able to find the link/address for a US distributor of replacement lens and related sealants. Back when I rebedded my CS 36T hatches with a silicone kit with primer I bought from the distributor it all worked great.


It's a standard CS27 port light. No idea who made it; there's no label to be found. In fact, I can't even find a replacement in a dimension close enough, or I would go that route. It's approximately 48" long by 5-1/8" cut out, so totally non-standard, unfortunately.

If I can't bond this stuff into place, my fall back is awful in terms of both time and money. Laminating glass into the center of that cut-out to use smaller ports is the only way I can see to replace what I have. I'm so sick of glass work at this point that I'll need therapy if that happens. I'm on the tail end of a year-long refit at the moment and my mental health is pretty volitile


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## casioqv (Jun 15, 2009)

West G/flex epoxy has very strong adhesion to both aluminum and polycarbonate:

WEST SYSTEM adhesion data


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## LookingForCruiser (Feb 7, 2007)

Epoxy will not flex enough between aluminum and polycarbonate. They expand/contract at different rates, the epoxy will crack and it will leak.

You need something that flexes. 795 will flex. I don't know that it's the best choice or not for this project, but it's a better idea than epoxy.

I will be bedding my acrylic windows without primer, using 795. I believe that is safe to do so, but time will tell.

Make sure all mating surfaces are clean clean clean, use alcohol to clean up the edges of the aluminum & the polycarbonate (assuming alcohol is safe on polycarbonate), any oils from your fingers, etc, will cause the 795 to not bond properly and it will leak.


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## sailingfool (Apr 17, 2000)

cghubbell said:


> .... No idea who made it; there's no label to be found. In fact, I can't even find a replacement in a dimension close enough, or I would go that route. It's approximately 48" long by 5-1/8" cut out, so totally non-standard, unfortunately.


Try asking the CSOA and i bet you'll get the answer.


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## casioqv (Jun 15, 2009)

LookingForCruiser said:


> Epoxy will not flex enough between aluminum and polycarbonate. They expand/contract at different rates, the epoxy will crack and it will leak.


Are you sure? The g/flex is marketed as an extremely flexible epoxy- for bonding plastics. But I guess it's better to use a tried and true method than to try and engineer something new.


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## cghubbell (May 8, 2009)

sailingfool said:


> Try asking the CSOA and i bet you'll get the answer.


I'm on the CSOA list as well. They haven't made these windows since the days of wooden canoes. I've reviewed the dimensions from every port manufacturer I could find and nothing is even remotely close. I spend weeks over the winter researching this one.


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## cghubbell (May 8, 2009)

I received a frustrating response from Dow when I asked them for guidance. Because I used the word "marine" in my query, they refused to provide guidance, saying that they do not certify any polycarbonate for marine use. So, not even Dow is willing to answer the question about whether or not a primer is required when bonding aluminum to polycarbonate.

Meanwhile, I know for a fact that polycarbonate is used in architectural scenarios bonder to aluminum frames. Same UV situation as a boat, much higher winds hitting buildings...

Ok, the polycarbonate will fail sooner than lexan. I get it, and it's acceptable to me. Meanwhile, how do I bond it? Frustrating. I guess I'll just clean it as best I can and use the Dow 795 I have, hoping for the best.


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## LookingForCruiser (Feb 7, 2007)

casioqv said:


> Are you sure? The g/flex is marketed as an extremely flexible epoxy- for bonding plastics. But I guess it's better to use a tried and true method than to try and engineer something new.


You can ask WEST what they think of bonding polycarbonate to aluminum with it, maybe they'll say it's hunky-dory. Wouldn't try it without explicitly asking about that scenario. The spec page you linked didn't cover how much it would expand/contract before failure. Dow 795 will handle 50% of the original joint dimension.


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## GaryHLucas (Mar 24, 2008)

I bought the primer, it cost TWICE what the sealant cost! I had to buy a quart. I'd be willing to pour some in a metal paint can and send it to you if you are interested. I'm in Hightstown, NJ, in case you are local.

Gary H. Lucas


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## cghubbell (May 8, 2009)

casioqv said:


> Are you sure? The g/flex is marketed as an extremely flexible epoxy- for bonding plastics. But I guess it's better to use a tried and true method than to try and engineer something new.


I'd be more concerned with the impact it might have on future maintenance. You can scrape out most forms of cured sealant goop, but epoxy is always an adventure in grinding. That would not be an appropriate situation for the thin aluminum frames these panes mount in. One twitch when grinding and there'd be half the frame material gone.

It may well be that g/flex can do the job, but I don't think epoxy properties are well suited to the long-term maintenance of ports.


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## cghubbell (May 8, 2009)

GaryHLucas said:


> I bought the primer, it cost TWICE what the sealant cost! I had to buy a quart. I'd be willing to pour some in a metal paint can and send it to you if you are interested. I'm in Hightstown, NJ, in case you are local.
> 
> Gary H. Lucas


Not local at all (Rochester, NY here), but I certainly appreciate your offer. Probably won't need to to take you up on it though.

I just sent an email to Select Plastics asking for a quote. This isn't the sort of thing I want to have go bad, and the deeper I get into research the more obvious it is that using polycarbonate is a mistake. I may just have to eat my initial mistake in listening to an incompetent glass dealer.


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## mitiempo (Sep 19, 2008)

Polycarbonate is Lexan. The better choice is acrylic which is Plexiglas.
I would definitely not use epoxy. The Dow 795 (or 739) is a much better choice. With these you really don't need the frames. The trick is keeping everything in place until it sets.

My CS27 has the 3 smaller ports on each side and I am still debating how I will attach the acrylic (Plexiglas).


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## cghubbell (May 8, 2009)

mitiempo said:


> Polycarbonate is Lexan. The better choice is acrylic which is Plexiglas.
> I would definitely not use epoxy. The Dow 795 (or 739) is a much better choice. With these you really don't need the frames. The trick is keeping everything in place until it sets.
> 
> My CS27 has the 3 smaller ports on each side and I am still debating how I will attach the acrylic (Plexiglas).


Wish I had the one with three smaller ports, as that makes it possible to buy replacements from a few different vendors if you choose to do so. With the contraction & expansion of plexi, the shorter windows also have a better chance of staying sealed over the long haul.


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## cghubbell (May 8, 2009)

Had an interesting conversation with Tony at Select Plastics. apparently the window style on my boat s no longer ABYC compliant, and they won't do restoration work on it. His recommendation was to purchase acrylic and surface mount it using a process similar to what Don Casey uses in This Old Boat. 

His suggestion differed in that he recommends bonding the pane with Dow 795 rather than using the quantity of mechanical fasteners Mr. Casey suggests. Instead, only enough to secure the window during curing, then remove them and fill the holes with the 795. So, in the end the 795 is the only thing holding the acrylic to the cabin.

I'm going to go with this path. It seems to the be the only way to create a secure structure. The only place I'll need to get creative is the curvature of the cabin sides. Thanks to all who attempted to help me - I just seem to be a lost cause in this case!


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