To bring this thread back on topic, I've learned since I first started this thread that it was a mistake to use Sikaflex-221. Despite how others have used the product, Sikaflex-221 is really meant to be used as a weather sealant.
As per the
manufacturer's web site:
They do have a number of adhesive products, like SikaFast-3141 or -3131, but Sikaflex-221 is not intended to be used as an adhesive (see also
this older thread on the topic) and I shouldn't have
followed the lead of others to use it to adhere my panels. They may well be lucking out so far but my panels have a polyethylene backing which made them even less able to bond with this sealant.
I've been following up on suggestions made in this thread and looking into various mounting options which I'll summarize here:
1) Liquid Adhesive: This seems to be how many RV manufacturers (including Leisure Travel Vans) install flexible panels. Despite this, some folk recommend against using liquid adhesives given the potential difficulty of ever removing a damaged panel. With these polyethylene-backed Greesonic panels, it's important to find an adhesive that works well with "low energy" surfaces. One manufacturer recommended using
their SilverThane SA-2100 adhesive, but the polyethylene-backing would need to be flame-treated in order for this adhesive to be effective on that surface.
2) High Bond Adhesive Tape (3M VHB): This seems to be the most widely-used method of securing flexible panels. It's also how some well-respected installers, like AM Solar, secure the mounting brackets for much heavier, more air drag prominent rigid panels. AM Solar uses 3M's VHB 4950. I contacted 3M to find out which VHB tape they'd recommend for my polyethylene-backed flexible panels (a "low energy" surface) and they recommended VHB 5952. I've done some tests with this VHB tape using my damaged panel and it does seem to hold very well. It took a lot of force with plenty of leverage to get it to release just a 3" x 1" strip. Not surprisingly, the MCD board I was using gave up the bond first when testing against aluminum but the polyethylene gave up the bond first when I tested against that.
3) Dual Lock / Velcro Tape: Somewhat surprisingly, this is actually a reasonable solution. There are a number of industrial strength "hook and loop" or "mushroom cap" tape products that are used in demanding outdoor, mobile applications like securing flexible panels to an RV rooftop and others have found it to work quite well. It has the added advantage of being easier to replace the panels than with simple adhesive tape. The major concern I have here is that there does not seem to be as many options for the adhesive portion of the tape – and I need to bond with a very "low energy" polyethylene surface. Plus this will also create a thicker gap to deal with under the panels.
4) Screwing into the Roof: Like many others, I'm not at all keen to screw into the fiberglass roof of the Unity. It's easy enough to use sealant to prevent any leaks but it's not at all clear just what your screw will be grabbing onto and you'd be concentrating any forces acting on the panel on a few small points of the roof. It's also apparently not how most everyone else installs flexible panels. Lastly, you would end up having to reuse/weaken holes or make new holes if you ever need to remove and replace a panel. Notably, even well-respected installers like AM Solar use VHB with their rigid panel mounts rather than screwing into fiberglass roofs.
5) Countersunk Bolts through Flush Mounting Brackets: This is a tempting suggestion to use thin aluminum bars, metal angle braces, plastic brackets or something similar and then add upward-facing countersunk bolts. The brackets are secured to the roof with adhesive and the flexible panels are secured to the bolts through their corner grommets. This would have the advantage of spreading the load over a wider area than can be achieved with a screw into fiberglass and would allow the panels to be removed and replaced if they ever become damaged. This is similar to what some installers (like AM Solar) do with much heavier, rigid, framed panels: they mount the panels on brackets secured with 3M VHB 4950 tape (rather than screws) and then they cover the brackets in lap sealant. It would be necessary though to prevent galvanic corrosion between dissimilar metals like aluminum bars and steel bolts, perhaps using something like "Ultra Tef-gel".
So.... I'm currently leaning towards either #2 or #5. I'm still waiting for a stretch of warm, dry weather to proceed. I also need to decide between using sealant all the way around the panels to keep out moisture and prevent the panels from getting any lift or leaving a few gaps around the panel to allow moisture to drain. Different folks have chosen to do this differently.
I just wanted to update this thread now to warn folks off of following the "Sikaflex-221 as an adhesive" path.