There is more discussion about this in the archivs here, and in the shop tips section of clcboats.com, but basically the "1/4" luan has very thin surface veneers and the grain orientation of the thicker middle section may not be good for boatbuilding (it just is there to take up space). Some people have had luck with the "1/8" luan doorskins. I used 4mm okoume marine ply (BS-1088) and it seems to handle the compound curves well, as well as the scarf joints. The three plys are uniform thickness, with no voids.