It is established by spreading the pressure around the sheetrock in the area. You have six prongs between the two bolts. As the bolts are tightened the prongs spread on the backside of the sheetrock. This makes the sheetrock more stable. This allows the 600lb rating.
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Yes as they expand once inserted
It looks as though you can. I am holding an instruction page from inside a wingits package and it says "For 1/4" acrylic/fiberglass tub surrounds or plaster walls use 2 GBW40's" but I believe I would be reluctant.
How many fasteners in a package?
I used them on tiled walls where I was not hitting studs. Get 3/4 inch drill bit . Makes this job so easy. I put up 3 grab bars in an hour.
I would say no because it would pull the fiberglass to the wall securely. Maybe cracking it
If you can find the stud, then yes.
Yes, that’s the brand that I used them with. Just make sure that you follow the instructions, otherwise some of the fastener may be exposed and the snap cover won’t cover the whole thing.