Dow 795

[bcguy]

Hello: Just re-painted my steel hull with PPG 7-814C Line Int/Ext Gloss Alkyd. I have flush to hull plexiglass windows 3/8" thick I wish to attach. I could not find any reference to whether or not Dow 795 would adhere to painted surfaces. I would hate to have to grind down to bare metal.

..... Thank you in advance for your comments


Gary

[Bill O]

Gary,
We have used the 795 structural silicone on teak to plexi (hatches) and painted aluminum to glass (dodger) and both have worked well over the years.
The obvious weak link is how well your paint is adhered to the steel.

[slug]

Originally Posted by bcguy

Hello: Just re-painted my steel hull with PPG 7-814C Line Int/Ext Gloss Alkyd. I have flush to hull plexiglass windows 3/8" thick I wish to attach. I could not find any reference to whether or not Dow 795 would adhere to painted surfaces. I would hate to have to grind down to bare metal.

..... Thank you in advance for your comments


Gary

Some projects call for structural silicone so I use it

Its more difficult to work with because of long cure time
Seems to work well, no failures , standard “tooth “ surface prep


Silicone contamination can be an issue

In general I always use Sikaflex for window bonding

[CarlF]

I can’t answer your adhesion question but be sure that the bead of 795 is at least 1/4” thick wherever two different materials meet. This will allow for different expansion rates of the materials which will otherwise rip a thin layer apart.

When I set a skylight, I made a support of 5/16” thick foam weatherstripping tape to support the plexi while the 795 cured. This kept it from being squeezed out by the weight of the plexi.

Has been leak free for 4 years in Florida sun.

[slug]

Originally Posted by CarlF

I can’t answer your adhesion question but be sure that the bead of 795 is at least 1/4” thick wherever two different materials meet. This will allow for different expansion rates of the materials which will otherwise rip a thin layer apart.

When I set a skylight, I made a support of 5/16” thick foam weatherstripping tape to support the plexi while the 795 cured. This kept it from being squeezed out by the weight of the plexi.

Has been leak free for 4 years in Florida sun.

Yes indeed .. bond line thickness is critical

Foam tape can be used in many applications

Rubber adhesive dots are another option to creat a bond line

[Chotu]

Perfect advice above. I made all the mistakes with 795 first, then found out the right way, as described above

[bcguy]

Thank you for the posts... the "spacer" to keep the 795 from squeezing out is a very understated step (from my searching) much appreciated. I have never seen those rubber spheres but I will ask around paint suppliers here. The foam tape seems my best option as this is common everywhere. The previous width of the Sika Flex (lasted 14 years was 2 inches). Nevertheless 795 or Sika the "spacer" is critical (per posts). My question about caulking gripping the painted surface still remains. Since Dow has not covered this and indeed by answer of my email "admit" to not having done this research and have suggested "try a test". Well,.... I was expecting something more helpful, in sync with the flow of my project as I am currently in the thick it, scraping off well cured Sika. Maybe Sika is the way? But have heard so much about 795 on CF . Many thanks...

[bcguy]

P.S... just wondering about roughing up the paint with heavy grit sandpaper would be of adhesion benefit?

[slug]

Originally Posted by bcguy

Thank you for the posts... the "spacer" to keep the 795 from squeezing out is a very understated step (from my searching) much appreciated. I have never seen those rubber spheres but I will ask around paint suppliers here. The foam tape seems my best option as this is common everywhere. The previous width of the Sika Flex (lasted 14 years was 2 inches). Nevertheless 795 or Sika the "spacer" is critical (per posts). My question about caulking gripping the painted surface still remains. Since Dow has not covered this and indeed by answer of my email "admit" to not having done this research and have suggested "try a test". Well,.... I was expecting something more helpful, in sync with the flow of my project as I am currently in the thick it, scraping off well cured Sika. Maybe Sika is the way? But have heard so much about 795 on CF . Many thanks...

Foam is nice because it does two jobs .. creates bond line thickness and prevents caulking blow out on the inside of the frame

The rubber dots a best used around the perimeter of the window to keep the window centered in the frame

[bcguy]

Originally Posted by slug

Foam is nice because it does two jobs .. creates bond line thickness and prevents caulking blow out on the inside of the frame

The rubber dots a best used around the perimeter of the window to keep the window centered in the frame

Thanks.... just wondering do you run a 1/4" bead along both sides of the foam or "wiggly style" back and forth across the width of your caulking area overtop of the foam?

[Gary Mc]

So is the 795 used with, no mechanical bonding at all?


Can you use this for sheet acrylic ports to fibreglass cabin sides? That is how my windows are held in but I was not sure what silicon it is and wish to repalce the ports.

[CarlF]

I ran the foam tape around the inside edge (what is seen inside the cabin). I then filled the space beside the tape with 795 to the level of the tape. I placed the plexi and no 795 leaked past the tape. Finally I use an assortment of tools and sticks to push 795 from the outside to fully pack the space against the tape and then troweled it level on the outside - Mostly with a gloved finger. The finished job was very neat.

I cleaned all surfaces well with alchohol before applying 795. Four years in there is no sign of it losing its grip or looking “old”. It’s very tough and resilient stuff. Used to hold windows in skyscrapers with no mechanical fasteners.

Dow does make a primer. I didn’t use it but might be worth investigating. http://www.ssicm.com/assets/pdfs/SIL...rimer-c-os.pdf

[bcguy]

Thank you CarlF .... you have found a good solution for your boat windows. My 3/8" plexiglass windows mount flush to the hull. I like the reasoning behind the tape.

Gary

[slug]

The process for fitting a window is two stages

First you bond the window to the frame

Second ... Once the adhesive caulk of your choice has cured.... 48 hours or as stated in your technical document .. a finishing bead it run around the perimeter

These can be two different caulk products

The finish bead must have a high UV resistance and form well for a smooth bead

[Bill O]

For our teak hatches we did something similar to how Carl did his, but w/wood (which is part of the hatch).

We used 1/4" spacers to maintain the space as we set the plexi flush. We also cut the plexi 1/4" smaller (on all sides) than the opening and filled that w/the 795 when we initially set the plexi. Then as Carl described tooled the 795. So basically, the adhesive was a "L" adhering to the backside and edge to hold the plexi in place w/no mechanical fasteners.
Been this way for years and no leaks.