I thought it was worth it. The need to use filler blocks depends on how few bulkheads are used on the model. Some kits have so few bulkheads that the effect on the planking leaves the hull with facets in the curves that can only be fixed by using large amounts of filler in the flat areas between the hard bends where the bulkheads are. If you kit does not have a large amount of bulkheads in order to create smooth, organic, even curves on the hull, then filler blocks are needed.
The filler blocks become the shape which the planks assume, instead of relying on the natural bend resistance of the planks to create the curves. The number of bulkheads should increase in density (smaller space between bulkheads) where the hull has sharp changes in curvature, such as near the bow and stern
Sometimes planks have weak spots where they bend at a sharper angle, and thus do not match the curvature of adjacent planks, creating offsets between the surface of that plank and the ones next to it. Those offsets are usually not extreme and are removed during the sanding process. Filler blocks remove any offsets since the first planking layer is glued right to the surface of the blocks and the edges of the bulkheads. That saves on sanding and filling. Of course, you had the extra step of sanding all those filler blocks, but the payoff is that you when shaping the blocks, you get to see the precise form of the hull curves earlier, and make smaller adjustments with filler thank you would on a lesser supported planking layer when you didn't use blocks.
Of course the overall hull is stronger with filler blocks, and far less subject to punctures from accidents, and it is noticeably much heavier.
My first kit, Corel's
La Couronne, had a sufficient amount of bulkheads to ensure that the first planking layer created curves with few hard bends and flat spots, but two applications of filler and sanding were still required. The DeAgostini
HMS Sovereign of the Seas had far fewer bulkheads in the stern area and midships that were required to support the planking. So, I opted for filler blocks for that model. Because massive changes to the stern were needed, the filler blocks allowed free-form carving of the stern and complete reshaping from a square tuck to a round tuck. If there were just bulkheads and planks there, such changes would have opened up huge holes in the hull and made filling those holes and reshaping of the stern far more difficult.
Personally, I don't like filling and sanding. It's VERY messy, and you have to take A LOT of time gazing at each side of your hull in sunlight to make sure you filled in all the flat spots and making sure you kept the hull symmetrical from port to starboard before moving onto the next steps in the build. Investing in filler blocks saves time later in sanding planking, and shaping those filler blocks (also messy) allows you to get the hull shape close to perfect before planking even starts. Also, any mistakes in the shape can be spotted and sanded without worrying about sanding a hole in the hull, and depressions you made by over-sanding can be filled with glued on thin layers of balsa and re-sanded quickly.
The mouse sander with 60 grit sandpaper made sanding the balsa blocks down to the level of the bulkhead edges a very fast job.
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