What is this brown dust/powder inside my old pack?

Question

I have a Lowe Aline Attack 50 pack from around 2000 and recently much of the inside has accumulated a sort of brown powder that is either, I suspect, some kind of mildew, or the remains of the decomposition of the pack's waterproofing.

The stuff is not easy to remove; if I beat the pack while holding it upside down, the dust gradually falls out. Wiping it down with a damp cloth or cleaner doesn't speed the process up much. The stuff appears to be only on the waterproof liner and, to a much lesser extent, facing surfaces. What is it? Can it be removed? Is it just time for a new pack?

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Addendum-- I've already accepted an answer below but wanted to share that Lowe customer support concurs that the waterproof liner has decomposed.

Answer 1

As a tailor, I would say that it is either the water proofing is breaking down or if it contains a fabric glue binding two types of fabric together is disintegrating. This is almost impossible to cleanup correctly. Personally I would either use it as it is, if you could use it in some way that such a condition would not be a hindrance for your backpacking, or get rid of it.

You could consider a pack liner (like a large trash-bag inside the bag).

Personally, I would simply get a new backpack. Sorry.