Originally Posted by boywonder
......You are probably over-analyzing it a bit.
Well I suppose; but on the other hand, I figure if they make a variety of kinds, one or two must be better (or worse) than the others for this job. And I have to pick one. I like to understand why I'm choosing one vs. another. Also because I'm considering it for some other uses, I needed to get a general understanding.
The 3M technical document is really helpful (vs. a few of their "pretty" ad type documents that really weren't all that useful). It just took me a while to get past the density (not that I am all the way "past" that yet).
At any rate, my notes above basically cover what I know (although I was in a hurry when I typed it). The two tapes recommended by the 3M tech person for this job were either 4941 or 5952.
4941 leans more toward high surface energy and perfectly flat mating surfaces.
5952 also does high energy surfaces, but then leans more toward medium surface energy items as well (the adhesive) and less perfectly flat surfaces (the foam).
I think either would likely work fine - the 3M tech indicated such too (with a slight preference for the 5952 due to the one molded/gelcoated surface).
IIRC, I chose 4950 for my buddy's SMB (when I had only done a "normal" amount of researching ... gasp) and it has held fine so far. It would be to the "other" side of 4941 from 5952, in a way (extending less into the medium surface energy part of the scale). What I didn't realize at the time was that our gelcoated fiberglass (more specifically polyester resin) was lower in "energy" than, say, epoxy resin/fiberglass. They list the latter as higher energy, but then something like a two part polyester paint as medium energy -- so apparently polyester fiberglass is more mediumish. It's not way down there, but just lower than the high of aluminum or similar. For the roof of a painted van, 5952 would definitely be recommended (and not 4941) because polyester type paint is more toward medium energy. We can apparently use either one on a gelcoated fiberglass/aluminum bond. But since polyester/gelcoat is somewhere in the middle, the tech person leaned slightly toward the 5952, but also said the 4941 should be fine. 4941 is slightly less tolerant of high heat, but 5952 is only a bit better (200ºF vs. 250ºF, although either can stand more for short periods).
The tech recommended the 1.1mm thickness based on my description of the fiberglass surface (aluminum angle was obvious of course).
So just to recap, if anyone is still awake or cares
1) Check the two surfaces that are being bonded to see where they fall on the high energy to low energy scale (essentially high energy is hard; low energy is soft). 3M has charts in their tech document. This directs you to a particular adhesive type (there are three or four).
2, 3) I'm not actually totally clear on how these two differ from each other, but there is foam type (firm, conformable, very conformable), and foam thickness as two different factors. I think they both have to do with how "flat" the surfaces are, but haven't quite grasped the subtle differences. Maybe the foam type is more to do with the energy of the surface.
At any rate, I'm going to go with either 4941 or 5952, in the 1.1mm thickness.
Prep is likely the most important (as always, even though for some reason it is the least exciting).