My top tip is to use 100% wool that has been untreated (manufacturers sometimes treat wools to make them more machine washable). This can come in many forms:- In the past I have bought fine hand dyed wool and thick slub wool which have both given me very different felted finishes.
My next tip is to avoid man-made fibres at all costs even if they are heavily blended with wool. This is because at any high temperature the man-made fibre will often stretch and melt rather than buckle, crimp and tangle like wool causing knits to become loose and floppy. I have been told that small amounts of man-made fibre content (less than 10%) can work, but I avoid them personally (although this is more of my own preference).
You can also try blends with alpaca and mohair as these are good fibres for wet felting and so should work for knitted felts too.
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