Will using WD-40 to flush cable housing dirt harm the housing?
I will have to make many cuts to new housing and do not have a housing cutter tool or a dremel. If I can reuse the old housing effectively, that would prevent me from making bad cuts.


Before I was able to afford/invest in a more suitable (i.e. less crushing) cutting tool I used to make do with common side-snip pliers; then doing my best to reform any squashed portion of the coiled housing so as to not obstruct the opening at the the end.  

On top of this you also try to keep the face of the housing end as square as possible to the cable so it sits as neatly and unobtrusively against the barrel adjuster or cable stop at either end; minimising bend or creating a friction point.

Cut as short as lengths off as possible of course and use old waste lengths to do some practice cuts, knowing what you want to achieve.

It's doable; just a little bit more fiddly and irksome. I've been there.