G Flex epoxy will bond much more strongly to the ABS of Royalex boats than conventional epoxies like West Systems 105/205-206 or the Total Boat epoxy that was just mentioned. I have used it for repairs or installation of abrasion plates on dozens of Royalex canoes with excellent results. Before G Flex became available people did repair Royalex boats with conventional epoxies. Sometimes these repairs would be durable but I have seen many such repairs simply flake off after a year or two. Part of the problem is the compliance mismatch between conventional cured epoxy and the Royalex material. Royalex flexes much more than cured epoxy and the repeated sheer stress eventually leads to failure of the bond. I have removed many fiberglass patches applied to Royalex canoes with conventional epoxies by simply pulling the fiberglass patch off with my fingers.
Plain weave fabrics up to about 6 ounce per square yard are really not hard to wet out with G Flex unless you are working in very cold ambient temperature. I have applied many patches and abrasion plates with 5 oz/sq yd aramid cloth, 5 oz/sq yd Dynel fabric and 6 oz/sq yard fiberglass with no issues. I would not try to wet out multiple layers of such fabrics at one go, however. Always use unthickened G Flex to wet out cloth. I apply a thin layer of G Flex to the hull surface before laying on the fabric. The viscosity of G Flex does depend on ambient temperature. You can reduce the viscosity if necessary by gently warming the mixed epoxy but this will diminish pot life. You can also waft a heat gun or hair drier over the hull surface before applying your cloth and can do the same over the fabric to speed up the wet out process if you need to. I have found that it takes a little longer for the fibers of these fabrics to take up G Flex epoxy, but if you are patient I don’t think you will have any issues.
Fiberglass tape has a selvage edge on both sides of the tape which prevents fraying along those sides. That makes it easier to work with than plain weave fabrics which tend to fray. But if you are applying multiple layers you can not orient the fibers at different angles as is desirable. And once fully saturated the selvage edge tends to stand a bit proud of the hull surface and the rest of the fabric unless you sand it down.
As for how to proceed with your repair I would have to see the boat in person and work with it before deciding on the best approach. But in general I would definitely remove anything that is loose or easily removed that is not Royalex. That includes the fiberglass patch if it is relatively easily removed. You will want to cover an area of adjacent undamaged hull at least 2 inches in width in all directions with your fabric. If possible, remove the outer layer of vinyl from the Royalex in this area by sanding or using a wood chisel. Any vinyl you can’t remove needs to be sanded with 100-120 grit paper and thoroughly cleaned before your repair.
I expect you will find some exposed foam core after you get all the crap off. I fill in any exposed foam core with G Flex moderately thickened with colloidal silica powder (cab-o-sil). This usually takes a couple of applications as the epoxy settles down into the interstices of the foam core. If you overfill slightly you can sand the epoxy down fair and smooth after it cures.
After filling in any exposed foam core sand the entire area you are going to apply cloth to with 100-120 grit sandpaper. I would then wash the entire area with warm water with some Dreft dishwashing detergent, rinse very well, and clean the surface with denatured alcohol. I would apply at least two layers of 6 ounce/square yard fiberglass over any severely damaged area making the second patch one inch smaller concentrically than the first. And orient the fibers of the two patches so that the cross each other at a 45 degree angle. Completely fill the weave of the cloth with a second application of epoxy.
If you buy G Flex read the instructions carefully. You will see that West Systems recommends pretreating the Royalex hull surface by flame oxidizing it with a hand-held propane torch but indicates this is “optional”. Their test data indicates it does enhance the bond strength of G Flex to ABS but it must be done carefully to avoid overheating the Royalex and deforming the foam core. I would not try to flame oxidize any exposed foam core however, as this is very easy to melt.
I have most often used Krylon Fusion spray paint to cover any repairs although pretty much any paint will work. It will tend to get scratched off but is relatively easy to reapply.