I did a search for some prop., and read that some brands are sometimes dirty. dirty was described as being unsterile. it was this that would cause the site to be sore and painful. my question is can anything be done to clean it up so injections are not painful.
I don't think that filtering it is going to cause it to be less painfull , most guys will agree that prop is just a painful compound. Try mixing it with some EQ if you can get it , that is suposed to help.
After filtering it to further sterilize it I would put just the needle of say a 20 guage through the bottle, then put it in the oven for 10mins at 300 degrees. You can do this once or twice. The needle will allow hot air to escape so the bottle will not break.