I think you need an adhesive-sealant, one that adheres to both the fastener (your screw or bolt) and the matrix (the GRP and the fitting i.e. the traveller track, cleat, etc).
Depending on your context - meaning your geography and where the fitting is located - I think you have to consider environmental exposure (UV, temperature). Bedding compounds, whether butyl rubber or 3M's 5200, all age. UV and heat accelerate the speed they age.
Depending on temperature (decks and deck
fittings get hot), 5200 has an age limit - based on my need to rebed deck cleats
, my experience points to an average somewhere between 5 and 10 years.
Technology keeps doing a better job. In the 18th century, Stockholm tar was cutting edge. In the 1950s, Dolfinite was cutting edge. Both work, but there are better products available.
I would suggest you spurn both 5200 and butyl rubber as obsolete. Both work, but there are better products available. And of course not everyone has caught up with the latest.
I think the current
best technology has surpassed them in the form of silyl-modified polymers (SMP). No doubt SMP bedding compounds will be surpassed soon. But not yet.
For brand names, look at Simson ISR 70-03 or Simson MSR (ISR = industrial special range; MSR = marine
special range; a chief difference is that MSR has International Maritime Organization rating for being non-flammable).
Simson is a brand name belonging to Bostik.
Try your preferred search engine
* For MSR: Products -> Transportation -> Marine -> Simson MSR Deck Caulk
* For ISR 70-03:
Bostik Australia :Industry - Catalogue - Simson ISR 70-03
Depending on the economy in which you're located, you likely will not find these products at your corner store.
Bostik Simson targets 'the trade', not retail consumers. Internet
retailers (eBay etc) may provide. For ISR 70-03, look for auto windscreen repairers/replacers, because Simson ISR 70-03 is the adhesive-sealant of choice (remember the point about temperature? Windscreens get even hotter than deck fittings left in the sun).
Simson MSR was patented as a bedding compound for teak
decks (which should also tell you something about its properties as a sealant and adhesive coping with UV and a wide temperature range while staying elastic) - see attached patent US 7581505 (about 500 KB ).