A truely rotten record i bought 2nd hand a long time ago. i used to brush it with those carbon brushes, washed it with palmolive soap & soft cloth and also tried the “knosti” cleaning machine with that very record before i finally got me a Hannl Micro vacuum record cleaning machine (similar to Nitty Gritty, Vpi, Keith Monks or the really cheap KAB) and made some experiences: Nearly every record sounds a bit more brilliant after washing (also brand new ones sometimes) – Real scratches oviously can’t be repaired but: There is deep groove dirt which can cause pops or even skips that vacuum record cleaners can reach while other conventional methods fail! Here’s an example what a vacuum machine can do what a velvet brush or a dishwashing detergent can’t!!
Update: This is the by far cheapest solution, I guess with the same results but less comfortable, louder and you will need to buy a proper brush & some kind of flat soft underlay where you can brush & vacuum the records on