<warning - waffle ahead!!>
Rivnuts..... ahhh, wonderful things. I'm always amazed i managed to build my Sammio without them! That said! I have tried to use them in all sorts of situations over the past year or so on the Countach, so i can give you a few examples of how they work "in reality".
I've mainly used aluminium rivnuts as I've not had much need to fit anything onto steel and i was warned at the start not to mix dissimilar metals. But i have used one or two steel ones in steel tubing and they are a lot more permanent than the alloy ones and a lot harder to squeeze!
The aluminium ones i have used a lot in alloy sheet and alloy box to mount panels, i.e. inside my front luggage area and they work a treat. Although you have to remember that as they are aluminium they are quite soft and I've noticed with panels that i take on and off a lot (at least the brakes are now fixed!) they can loose their thread and i have had to replace a few already.
As PaulL said I have also used them in fibreglass, primarily to hold my rear air intakes in place as well as a few other bits-n-bobs.
While the fibreglass in that area is a good few mm thick (6 to 8mm) and non-visible what i found was while they are good to attach panels, if they are removed/refitted just a few times then it is the rivnut that comes loose in the fibreglass.
I have now had to bond a few in place to stop this and this behaviour seems to be quite common when squished (technical term!) as they can quite easily eat up the surrounding fibreglass. Although it's worth mentioning that there is no starring or cracking in those areas after a good year of being in place!
But you piqued my interest about thin fibreglass so i just popped to the garage and had a quick test.
I found a bit of 4mm fibreglass (that looks remarkably Sammio grill shaped!) and fixed a 7x12mm rivnut into it to see what would happen!
i didn't have to squeeze too much to get a very firm fixing and on the front side it looks fine with no cracking
On the rear side it also looked really good and you can see that it doesn't have to expand massively in order to get good purchase. I also couldn't see any sign of cracking when i was in the garage.
Although.... when i came down and was uploading the pics you can clearly see on this one that the backside is stressed (i really regret writing that on this forum!
)
That seems to show some stress around the rivnut, but seeing as it's only been in place a few mins i've no idea how that would develop over time? TBH i'd not want to mount anything heavy on it!
Best of luck with whatever you do, just post pics so that we can all have a good ponder (and steal any ideas that work!
)
Dave