To clean the end for remaking a loop on a dirty fly line use a little nail varnish remover or acetone on a cotton pad, this softens the surface a little as it cleans and when heating it to fuse together, makes it a lot easier and more reliable.

Lines get brittle when they lose the plasticiser that is incorporated in the making of them to give suppleness, some of the commercial line treatments will replace this but not all.

Do NOT use Methyl Ethyl Ketone (a plastic gluing preparation solution) - this will disolve the line completely.

As it is only 3 inches you would be better to remake the loop instead of trying to repair the nick.

TLs