Is Baltic Birch Plywood Similar to Aircraft Plywood

Using baltic birch as a prototype was probably made up of two other thicknesses. However it could have been done by the manufacturer as a special order. This is not usual of all plywood building products. The baltic birch plywood will often have thinner veneers boards and denser fibers than marine based plywoods making it more suitable for a small part as you've described but you will need to be very finicky about sealing the grain otherwise black staining and eventually rot will destroy the part.

 Using Epoxy products will be your best bet but you will need to ensure that you select one with a hardener that is formulated with UV inhibitors. My choice would be West System 105/207. The sealing process is referred to as "encapsulation" and usually requires three applications of epoxy plus using a UV resistant finish such as varnish or polyurethane. Marine grade plywoods will withstand the weather much better and if non-corosive fasteners are used such as stainless steel or bronze is much less likely to suffer from staining or rot but diligent sealing will still be required for longevity. While BS1088 plywood would be best, lower grades could still suffice for this application.

Another choices may be limited in 1" thicknesses. I can't comment on using aircraft ply but the same comments for baltic birch plywood would probably apply.  The finishes will deteriorate over time and clear finishes are the most susceptible. Detailed initial application will ensure the greatest life for the product but timely maintenance is equally important.

Chances are Re-coating has to happen before the existing finish breaks down. Only once you see any cracking, peeling or clouding, you are too late and stripping down to bare wood is required. Lastly, most problems with parts such as this occur in the places that moisture can get to but cannot easily get out of such as where the part is mated to another surface. In marine construction we deal with this by using bedding compounds in these faying surfaces. These are usually non-curing oil based products similar to window glazing putty or rubberized caulking products. It is especially important that the fastener holding the part on is coated as it enters the part. Household products will suffice for this application.