I sell a limited selection of strings and install what I sell. Below is a selection of strings I recommend.

Evah Pirazzi	D'Addario	Pirastro	Thomastic Belcanto
Thomastic Spirocore	D'Addario Kaplan	Obligato	Pirastro Oliv
Pirastro Permanent	Innovation	LaBella Gut	Zyex

Try cleaning the strings with a small amount of rubbing alcohol on a bit of clean rag.

Keep rubbing string with a fresh section of rag until the string wipes clean.

If the sound of the string does not improve after cleaning, it's time for a change.