But if the hairline crack doesn't break the paint film the oil will
still be contained. Besides the oil, weld a boss on a longeron tube end
and install a tire valve and pressurize to 100 or so psi pressure during
the annual. The pressure should be enough to make the paint film fail
and cause leakdown. Find the leak by listening or water/soap.

Llama helicopters charge the truss permanently with dry nitrogen and
have a pressure gauge that is in view behind the cabin and can be
checked on a DI.

John

Now that is an interesting way of doing it. Just
check the pressure on preflight, and you'll know
there aren't any cracks if it's still holding
pressure. Plus, if charged with an inert gas such
as nitrogen, helium, or argon, you also know there
will not be any corrosion inside the tube.

Don W.