I used colored epoxy
I used a rather fine-gauge syringe with the plain old west system resin/hardener I used on the fiberglass. I found it difficult to see how much epoxy was going in the hole, so I used some colorant (comes in a tube from West) to give it some opacity. Since my rivet puller tended to leave a burr on the mandrel hole, I gently demurred the hole before filling with epoxy. If the epoxy is a little too high, it can then be easily sliced off with a blade after curing. (If there is a burr, it makes sliced the excess off a real pain). It sounds, and is, tedious, but I would guesstimate I did the exterior rivets on the entire plane in about eight hours, and Iím quite pleased with the results after painting.
RV-12 #120332 N73HR