Unfortunately, some porous cloth does not show tiny holes when held up to the light. Not only that, some cloth looks fine and is tight, for a month or so after application, then begins to leak!!! I assume that this is due to the cloth being stretched over the boards, thus straining the low-grade rubber to the point of microscopic breakage. This is particularly exasperating, although usually the defect can be found right away.
The only reliable test I know is to glue the material to an airtight structure, then test under pressure. A bellows or pneumatic should be simply impossible to collapse without damage, if its outlet is plugged. If it slowly or rapidly closes, something is wrong.
This reminds me of another subtlety in the world of airtightness:
When doing museum work several years ago on a "carte blanche" basis, in the pursuit of perfection I decided to perform suction tests on the shellac membrane with which manufacturers sealed wood in the old days; for example, valve wells, manifolds, etc. In almost all cases, I found it highly porous!
It quickly became apparent that in many cases, all manifolds and borings in wooden chests needed to be filled to the top with shellac, then drained, as most player action manufacturers did. (I disliked using other sealants, for reasons of authenticity and -- because I had seen how previous applications of lacquer, etc., by other restorers, simply sat on the surface of the old shellac and did not bond to it.)
I first made a test block, by drilling a large hole in wood and covering it with a carefully glued pump-cloth patch. I then applied mouth suction to an outlet I had provided, through a rubber tube. Compared to the suction I could achieve by pinching the tube, that achievable in communication with the wood block was negligible.
Next, using a can of shellac (3 lb. cut) from the store, I filled and drained the chamber in my wood block, and allowed it to dry thoroughly. Compared to the pinched tube, it still was not airtight! Finally, a third application of shellac rendered it "fairly" airtight.
I was very displeased with these results, so I contacted a chemist at the William Zinsser Co., makers and suppliers of shellac. He urged me to make my own shellac from flakes (which they sell) and denatured alcohol.
After doing so I repeated my test. A single application of this homemade shellac produced 100% airtightness. I remember showing this to another well-known restorer at the time, who was amazed at the difference.
The reason is that shellac at stores is usually simply too old to produce optimal results. It must be made reasonably fresh.
I have chambers in wood I sealed in this way in 1980, and the shellac looks as good and is as airtight as it was then.
Although it obviously does not last forever, I still believe that, for most restoration work, flake shellac, preferably orange (for the outside of most striking pneumatic boards I use white shellac, identical to that used on most originally), is the ideal sealant for wood. It marries perfectly to original shellac and remelts it slightly. It is bound to have a satisfactory life span (outlasting cloth and leather, etc.) and that is good enough. Shellac remains slightly elastic for many years, thus it will not crack away from its moorings. It also sticks well to metal. (For new work, I see no reason why other, modern sealants would not work as well or better).
Shellac is highly unsatisfactory as an adhesive or filler unless specially prepared. Just burning off the alcohol does not make a good filler or adhesive product. If all surfaces are clean, high quality shellac can be used thick, as with "pad cement" used on woodwind instruments. For certain player work, such as cementing in brass or nickel-plated brass nipples, I recommend an amber-colored product called "3M-847," which is flexible, sandable, quick setting and looks and behaves precisely like thick shellac, only better. I have used this for over a decade and therefore have some experience with its behavior over time, etc.