Spectra line does not melt, the sheath does but the core will not.
I used to work in a climbing shop and cut the stuff for customers all the time. You need to cut back the core and the melt the sheath around it sealing the core.
Spectra and its cousins are for climbing applications such as creating anchors. It is not ok to use as a prussik. The Maxim product is a little different in that it can be used for friction knots.

All in all 550 is a better all around choice.