I own a Shop-Vac "QSP" series shop vacuum. I bought it used from eBay (it was actually being sold by a pawn local shop, so I didn't have to have it shipped)
It has good suction power with a fresh filter, and unlike most shop vacuums I've used, it doesn't hurt my ears the way most shop vacuums do; the highest and most piercing frequencies have been muffled. This is good - many shop vacuums are loud enough and piercing enough to cause real hearing damage. I've seen units which owners have fitted with sound-containing housings around the blower motor. That's all that the makers of the newer, quieter units are doing - adding some plastic and a bit of foam rubber to absorb the high sounds while letting the air flow around corners in the muffler housing to get out. You can DIY that sort of thing if your unit is older and louder / more piercing.
My one complaint about my unit has been its poor air filtration and dust leakage, and the fact that most of the filters I had tried clogged up too quickly, killing suction too quickly. Until recently, every filter I tried either did not filter fines very well, or if it did, it clogged up and had to be cleaned frequently. More expensive, user-cleanable (not disposable) filters saved money, but cleaning them was incredibly messy and tedious.
Recently I discovered "drywall filter bags", intended to be used inside any typical shop vacuum. These are bags made from reinforced filter paper, not terribly different from the ones many household vacuum cleaners use, except that they are much bigger. This product was developed for cleaning up drywall dust during or after sanding, one of the messiest household (or contractor) cleanup jobs there is. Drywall dust clogs most shop vacuum filters very quickly. The bags have more surface area than any cylindrical filter could, they are fairly easy to put in and remove, and they completely contain very fine dust. They have a rubber gasket which fits over the intake port of the vacuum cleaner's tank to prevent dust-laden air from going around the filter bag.
Recently I also found a shop vacuum filter claiming to be a "HEPA" filter. "HEPA" is an acronym for High Efficiency Particulate Air, and it has a specific definition, at least in the USA. To qualify as HEPA grade, a filter must remove 99.97% of all particles larger than 0.3 microns (micro-meters, or millionths of a meter) from the air passing through it. That is a non-trivial task, and quite frankly, the vast majority of air filters being sold to consumers as "HEPA" grade filters do not actually qualify. Read the manufacturer's own claims of particulate removal and see if it matches the criteria above.
Even if these filters aren't true HEPA grade, they do a REALLY good job of removing very fine dust and allergens. So a few weeks ago, I dropped about $30 for a close-to-HEPA grade filter, and about $16 for three disposable drywall filter bags. The HEPA filter mounts on the vacuum motor lid in the usual manner, and the drywall bag wraps around that filter and extends below as well, filling the entire tank. Whatever the drywall bag doesn't catch, the "main" (now secondary) filter does, and airflow restriction is nearly nonexistent with this setup. When the bag is finally full (ie; when the suction finally starts to drop), you just pull out the bag leaving the tank completely clean.
The air coming out is clean enough not to trigger my or my wife's allergies - I can't even detect the faintest hint of dust.
Obviously this set-up is for dry vacuuming only. If you need to do wet vacuuming, you need to use the specific filter type intended for that job. I've yet to see a very good filter for wet vacuuming, most of them are simple foam sleeves.
PS: most regular cylindrical shop vacuum filter have a foam rubber gasket which should seal air- and dust-tight around its mounting position. I have found that if that plastic sealing surface or the gasket on the filter are not clean, fine dust can leak past. Wiping the dust from the sealing surface with a damp rag prevents that issue. Once you've started using the above setup, you won't see nearly as much dust collecting on those areas in the future.
Hope these tip help someone else.
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