I use West Systems Epoxy which is already thin by its nature and stronger than most hobby store epoxy.
I got into using the West Systems with my other hobby, high power rockets. Some of the airframes had to be fiberglassed to withstand the strains of flight. ie.. extreme G-forces and transitioning through transsonic into supersonic flight.
Until a couple of days ago I thought West Sytems was only available in 1/2 gallon resin and 10.6 oz. hardener (it is a 6 to 1 mix) but my local marine has much smaller sizes.
I bought the 10.6 oz hardener, because I was out, for $30. might seem expensive but it will last me several years.