A good problem preventing practice to undertake while building or rebuilding a tube and fabric aircraft is to treat the inside of the tubes either before or after construction to prevent corrosion.

For aluminum tubing I recommend sloshing with Stewart Systems Primer/Sealer. It is an excellent corrosion inhibitor and seals against moisture penetration.

For steel tubing that already has a little rust inside I recommend Stewart Systems Steel Conversion Coating. This would convert the rust to black ferric iron and then seal it in a clear acrylic coating.

For new steel tubing I recommend Stewart Systems EverShield Ceramic coating. Once sealed in this thin layer of ceramic not even standing water will be able to penetrate through to the metal. Because it is such a thin material and totally inert it will not affect welded repairs like oils will.

Speaking of oils I have found that oil in tubing is worse than nothing at all. If the airframe should accidentally be submerged in water the oil slowly floats to the top of the water pocket (like an oil slick in the ocean) forming a sealing lid on top of the water pocket, actually holding the water next to the metal causing terrible corrosion problems.

If you are not going to treat your tubing, (I hope you live and fly in a dry climate) I recommend drilling a single small hole at the lowest point on the airframe to allow the tube to breath and drain.  I have yet to figure out how water gets into the tubes to begin with.

If you are planning on rolling on your top coat rather than spraying, read the roll-on guide (PDF).