I've been browsing local plywood shops in the hopes of finding a local supply in Portland, Oregon.Regarding 1/4" for ribs on a Maupin Woodstock sailplane, Douglas Fir was called for. Here are my local options and looking for opinions and experiences with these:1. 1/4" marine grade A-B Doug Fir : I found it to be much more flexible than anticipated.2. 6mm Aquarek marine ply : uses mahogany I believe.3. 6mm Okume marine ply 4. 6mm Hydrotek marine ply5. 1/4" Russian Birch ply : very nice ply using exterior grade glue and appears to be about grade A-B. Not finding evidence it has voids and some 4x4 sheets had no patching at all.That's what I have found locally thus far. I know I could pay $$$ to have stuff from Aircraft Spruce but want to evaluate my local options.Who's got their 2 cents on these?!
I will offer the opinion that you want the marine ply. Those plywoods are manufactured and tested to real standards. I recall that the Aircraft Spruce catalog says something about the standard that their mahogany (Okume) plywood conforms to. The Russian Birch is great for cabinet work. You may find a quality stamp on it. If you do, look up the standard that the stamp represents. I think that you will find that the standard requires that voids must be patched and that while the required quality level is high, it is not quite as high as the marine grade. I will also note that the characteristics of Birch are a little different than Doug Fir. Not sure if it makes a difference in your application. I use Russian (Baltic) Birch in cabinets.
I would not be afraid of purchasing locally if the material meets a standard. Out here near Boston there is a local lumber yard that adveratizes aircraft grade plywood. Much rather spend my money locally than to pay for shipping, especially of larger sheets.
Best of luck,
Use the marine plywood. Why take a chance with the other stuff?
When I built my Hatz biplane I purchased a good deal of Finnisyh Birch ply from B&D International in Tacoma, WA. Their Ply comes in meteic thickness' and dimensions and is absolutely perfect. They can ship if you cannot come to the warehouse. Go to this web site, they also have a spec sheet on the ply. Barnstormer NX500L