Hi Patrick,
My experience with Wirraway frames, I would go as follows:
Get rid of any loose paint and dirt on the frame and take it to an NDT shop for xrays and at least have the engine mount attachments, wing attach and rear fuselage attach points. Obviously, the low points in the frame will be a good place to look for internal rust. The Wirraway frames I have seen xrayed (including my own) have come out pretty well, even though the frames have been sitting in paddocks minus wings, etc. Do any repairs before blasting.
Before you blast it with any sort of media, I would recommend getting a box of sacrificial pk type screws and block every hole you can find in the frame. Being a North American design,(I'm sure Patrick already knows this, but for all the other boys and girls out there) all the tube clamps have a sheet metal (pk) type screw through them to prevent the clamps from rotating on the tubes. If you can, enlist a buddy to go over the frame to find any holes missed. You do not want any blast media entering the tubes - you won't be able to get it all out. I also wouldn't use soda on these steel frames, as I understand you should wash it off with water - not a good idea in a steel frame that has a bunch of little holes drilled in it! Once blasted and inspected again for cracks, etc, I would then paint the frame. Once the paint is dry and you are happy with it, then fill it up with oil. Believe me, if you oil it before painting, you will never be able to stop the oil leaking out of the screw holes -screws or no screws fitted! On the Wirraways and I am sure the Yale and T-6 should be the same, there should be a screw going onto the rear flat face of the upper rear fuselage attachment. This is the best place to inject the oil. I will try to take a photo tomorrow and post here. Wirraways and T-6's originally used Linseed oil and it was generally heated up before injecting. This has two functions. First, it makes it more runny and secondly, you can feel the tubes warm up as the oil trickles through. This is a good way of ensuring that all the tubes get treated.
Have fun! It's a great looking project - I wish mine was that complete - and it should come up looking a treat!
