When building boats, I often have to round edges of various parts so that they are less prone to breaking, and less likely to hurt anyone who might hit against them. However, more often than not, these edges are located in tight places, often near inside corners or other structural elements, which completely prevent usage of even a small router, block plane or a sanding block. I have made several very small (5 cm) long sanding blocks with a groove and a sandpaper strip, but even the coarsest sandpaper takes forever to round an edge to a large radius, and the lack of accessibility of these edges makes this kind of work very tiring and inconvenient.
I do try to round the edges of parts before installing them whenever possible, but sometimes that can only be done after they are already glued in. My question is, what other tools can I use to round these edges even in the tightest and most hard-to-reach spots?
EDIT: I thought I'd add a specific example. Take a look at this photo.
It shows the inwale on the sheer of the boat (the railing with spacers). Every edge, inside and out, has to be rounded to a significant radius, so it is comfortable to use to lift and carry the boat without hurting fingers. However, a router cannot reach these short lower segments, because the side of the boat is in the way. A plane or sanding block cannot either, because these segments are too short. And this piece cannot be rounded prior to installation, because of other restrictions. The only way I can round these corners right now is with a steel file, held at a very awkward and uncomfortable position, taking a very, very long time at each segment. Surely there must be a better way to do this.