Whats your method of cleaning the underside of playfield plastic inserts?  Some times there black.
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As mentioned, cotton buds or a small artist/kids paintbrush works a treat as well.  I typically use window cleaner as the solvent - it doesn't bubble up and evaporates as well so it reduces the amount of drying needed.  The bristles can get into all the nooks and crannies.  If possible I use a dry brush first to remove as much dirt as possible before wetting it and turning it into a mud pie.