Any particular advantages for choosing marine plywood other than cost?On the Cygnet I am building Sisler calls for 1/4" marine plywood, no face designations, but no core voids larger than 1/8" and no patches within some distance (don't recollect the number at the moment) of the main spars. I purchased Roseburg marine ply from Menards in Owensboro, KY. I've cut all my ribs, haven't cut spar doubler plates yet. Bought 2 4x8 sheets. There were just a couple of voids in the two sheets. One less than 1/8" and one slightly more. I had 1 rib that ended up with that larger (3/16") void and plan to do what was said above and slide in a sliver with T88 epoxy around it.
In addition to the marine ply Sisler also indicated that clear spruce or pine was acceptable for the caps on the false front spar and the rear spar. The front caps are 1/2"x1/2"x156" and the rear caps are 1/2"x5/8"x156" on 3/32" mahogany aircraft ply webs. The lattice on the wings is also called out as clear spruce or pine and it is 5/32"x3/4" material cut to length (about 72"). Lots of room for material interpretation/substitution here. The main spars are called out for 3/4"x5 7/8"x156" spruce. Mine are ash of the same dimensions. The wings are over designed/overbuilt for the airplane if anything and very interesting.