Now that the lower fuselage joint was essentially bonded together, the next step was to strengthen the joint. This is achieved by filling the “valley” with rigid foam core, then glassing over the top. For this project, the Builders Manual called for medium density 3/4 inch core. Although lower density than the 1/4 inch stuff I was using to make bulkheads, I found this foam more difficult to work with. It was difficult to sand, although responded fairly well to rasp files.
Trying to fabricate one long piece of foam to fill the entire ~10 foot long valley would be have been ridiculous, so I attacked it 16 inches at a time. Each 16 inch (approximately) section of core was carefully filed, shaped and sanded so that it fit exactly into the valley. This was a very iterative and time consuming process of shape-test-shape-test-shape. Once I was satisfied with the fit, the foam section was bonded in place with a wet mix of epoxy micro, with some dead weight to keep it in place. I prefer Play Sand for this job.