My formative hiking years were in Washington, so prolonged rain was an expected "feature." It was technically possible to waterproof our external frame packs by sealing the seams, as they were relatively simple construction and had big flaps covering the single top opening. Combined with waterproof stuffsacks and sometimes, a poncho, our stuff actually kept dry. Pack evolution and the addition of features (like fancypants zippers) and then, internal frames made waterproofing them a hopeless challenge.

At present I know of truly waterproof packs with bonded, not sewn seams from ArcTeryx and Ortlieb. Almost nobody else bothers because it's so technically challenging, and requires "decontenting" most of the gizmos and features we've come to expect. They're necessarily big, mostly pocketless, single-compartment rucksacks.

Liner, cover, poncho? Yes. Which, depends on the conditions and the pack's design. I don't much like ponchos but for keeping water completely away from the pack itself, nothing is better. A cover plus a liner (compactor) plus a large-brimmed hat is a decent alternative. I've completely switched over to tape-seam rolltop dry bags for my bag and clothing, the last line of defense. Even if you use a liner, you still need to protect all the stuff in pockets. My packs have as many as six or seven.

Cheers,