There are many options open to you depending on the application. Adhesive selection is just one part of it, joint design and fibre orientation at the surface and surface pretreatment all have an effect. If you e-mail me with some details I can help you further
Backing rings are specified and used for the following reasons;
1) Improves installation time to compensate for poor alignment of valves etc. Especially when retrofitting into metallic pipelines.
2) Is usually cheaper than full face flanges especially when dealing with 400NB and above even more...