Aircraft Spruce used to sell an inside tube sealer that seemed to be a mixture of linseed oil and shellac. It was installed by hypodermic needle into holes drilled by a #55 drill into each tube. If drilled before welding they made a pressure relief port that allowed the welds to be finished without blowing back. There is a formula for the amount of sealer to put in each tube. The holes are plugged with tiny drive rivets, available at McMaster Carr. The frame must then be rotated in such a way that the inside of the tubes is covered completely. The sealer will fill small pin holes. I did a 4130 tube aircraft fuselage years ago, but didn't seal the tubes in the lakester because there will probably be more welding and it may be like a bunch of pipe bombs with the sealer inside. I did drill the tubes for welding however.
Harvey