Aluminum to fiberglass adhesive caulk?

Maybe some of y'all might have done recommendations for me.

I'm specifically thinking of you boating folks who have done rebedding of cleats etc, but anyone with applicable experiences is welcome to make suggestions.

Im planning on how I'm going to attach 1550 watts of solar modules to the roof of my Chinook. Based on standard wind load analysis, uplift forces could go as high as 4500 lbs, so the attachment system needs to be very strong and also spread out.

The roof is two layers of fiberglass, each about 0.2 inches thick. Sandwiched between them is a mix of 3/4 inch plywood, 1/2 inch ply and foam.

I'm going to mount the solar modules on 8 to 10 ft aluminum channels running lengthwize, placed over the 3/4 ply areas as much as possible, but in some areas they will have to be over the foam sandwich. I plan to use multiple stainless steel screws through the top fiberglas layer into the plywood where possible as the primary attachment system.

But, over the foam areas and everywhere else as a secondary attachment I want to use adhesive as well. Hopefully the selected material will also provide a good water seal, but I'll probably also use some Dacor lap sealant around the aluminum rails afterwards unless that would react negatively with the adhesive.

I looked at VHB tapes but rejected them because 3M specifies 15psi pressure to activate the adhesive in that stuff. No way I can see to get that on a rigid 1.5 inch wide by 8 or 10 ft long piece of aluminum, that's around 2000 lbs. Also, I had a very bad experience once with that stuff on a large solar project so I don't trust it.

So maybe 3M 4200 or 5200? Both are intended for marine applications, and are sealant/adhesive combos.

While I don't plan on ever removing the rails, who knows what might change, so I don't like the 5200 idea much because by all accounts it will never come off.

If you look at the numbers 4200 should provide around 200 psi adhesion which is way more than enough. And you can probably get it off if you really need to.

There is also a Sika equivalent polyurethane, I think it's 291, that got as good or better ratings in testing by Practical Sailor I read.

I used 4200 once to re-bed a couple of forestay attachments, but no real extensive experience.

With any adhesive, the devil is in the details, so I'm looking for specific info on fiberglass and aluminum surface prep dos and donts, application process, temps, thickness, etc.

Or Sika vs 3M vs something else. Any alternative ideas are welcome too.

Some of this info might even be useful for gluing stuff to rpods too...

Thanks in advance!

Thanks GG. The gel coat is pretty weathered on the roof and I don't mind abrading it further where the rails are going to attach to ensure adhesion. Sounds like that will be needed.

After decades of solar support structure work I approached the wind load analysis the same way I'd do an equivalent fixed structure. A fixed solar array in a 100 mph wind is going to see pretty much the same wind pressures as one on an RV travelling 60 mph into a 40 mph headwind. For that matter, I'd want to have the solar array stay on there if it's parked in a 100 mph wind anyway.

That case is handled by the structural engineering standards, and generally results in around a 25 pound per square foot uplift (or downforce) on a horizonal array with design wind speed about 100 mph. Unless you have wind tunnel or FEA data that proves otherwise you don't get any credit for fairings or other wind cheating devices.

Anyhow, that 25 psf is pretty conservative, and that's how I'd like to design this so I never have to worry about it coming off there. I think it's doable with screws plus the right adhesives, but I need to get all the details right.