Rear arches have always been on to do list for many years, what I use ( tigerseal ?) to stop any water getting in until it gets cut out properly over winter that wont make a mess getting it out . Sent from my SM-A405FN using Tapatalk
I would go for something like the fibre glass reinforced filler because it comes off in sheets with a bit of persuading rather than PU mastic that might get behind things, and end up very stuck to the last bit of clean paint or metal. Sometimes the best thing is to leave the hole, as it will dry out quicker than if you seal it over and really only end up trapping water for days. Its why products like POR-15 are so brilliant when new and so awful when failing as the water gets trapped behind. Either that or sculpt a surface out of Waxoyl underseal. It can be shaped by hand ( use gloves ) and can provide a waterproof surface. Could use that to protect the metal at the edges of the fibreglass and over the fibreglass. Some of my early bodges like that are still working 9 years on...
