I have been using Aeropoxy and Rocketpoxy for structural joining of rocket parts. Is there anything out there that has a low viscosity but is still strong? I'm trying to push this epoxy through a syringe with an 8 gauge dispensing needle on it.
Just about any epoxy out there will have lower viscosity (will flow more easily) then Rocketpoxy.
I usually fall back on West Systems 105 + 205 for larger jobs, and BSI 30 minute for when smaller quantities are needed. Both are clear (vs. black and gets everything black Proline), flow equally well, and are easy to inject through a syringe.
I would suggest ditching the needle for injection, and poking the tip straight into the injection areas. Needle is too much of a flow constrictor, and creates yet more parts that need cleaning afterwards.
BTW, if you are in a pinch and only have Rocketpoxy on hand, warming up its ingredients in a microwave before mixing will drop the viscosity and make it almost injectable. Higher temps will speed up the epoxy cure and reduce working time. Working time for Rocketpoxy's gets reduced in half from 30-40 minutes. Not ideal, but doable, in a pinch.