Another alternative is to use expanded bead foam block instead of balsa blocks. This is the foam you find in molded packing material.
Its primary advantage is that it is about half the weight to the lightest balsa.
It has some significant disadvantages though:
- You have to be careful of the glue you use. The Gorilla polyurethane glue works great. Quick-Cure Gorilla Glue, 2oz. Bottle is very convenient.
- You can shape it with fine sand paper, but sometimes no matter how carefully I go, one of the beads pops out leaving a little hole.
- You probably want to glue the unshaped block in place before you start sanding.
- When you cover it with tissue paper, is brighter than other areas of tissue that have nothing behind them or have wood behind them.
- Most paints will destroy the foam. You can seal it with Future Floor wax (actually an acrylic product) and then paint it.
- it is very soft (plan on battle damage if you fly the model)
I used in on a p-51 from the guillow 500 series and I liked the results.
Still another alternative I have been meaning to try is to vacuum form the wing tips. I actually purchase a old Mattel Vacuform on ebay, but I have never got around to making wing tips and then comparing their weight to foam and balsa block.