are all used LPs dusty?

Some are not but it doesn't matter.

If you want good results with used records, you have to be prepared to clean them properly first. There is lots of info here on A'gon and elsewhere already regarding various effective ways to do this.

Otherwise, stay clear of used records in general. Its hard to tell by visual inspection if a record is really clean or not.

"Wetting without vacuuming away the sludge just rearranges the sludge. "

True vacumming is the most common solution I suppose.

I have found that using tightly folded edges of a very soft and absorbent paper towel like a record cleaner while the record is spinning does a very good job of removing excess solution when cleaning a record.

I've been doing this with good results using my Linn Axis table now for a good 25 years or so.

I use extremely dilute Shaklee Basic H cleaner in water, distilled water is preferred if availble.

I spary the record surface down thoughourghly, place teh record on teh table, give it a good scrubbing with an old discwasher brush, drying the discwasher brush periodically as I go using another absorbent paper towel as needed.

Then I use the paper towel technique above to dry the record.

Once you develop the technique, is is inexpensive and most effective with only one pass and only takes a couple minutes per record side.