When to cement airplane canopies

It depends upon the project. But I pretty commonly glue at least the windscreen in place so any seam filling and clean up is covered by the paint. Canopy hoods I tack glue in place usually as well, but will remove them after painting and probably pose open upon completion. 

When your doing a paint scheme where the canopy has multiple colors on the framing because of a camo scheme or whatever, it is best to glue the canopy in place before painting for color alignment.

Each build is different.  If I am building a closed up canopy I tend to glue before painting.  With open canopy builds I have done both paint the canopy separate and on the plane. Sometimes it can be a great way to cover the cockpit during the paint process.

I usually will roll blue tack into very small worms, place on area where canopy would touch fuselage when closed and then press the canopy in place.  This secures the canopy for painting.  Of course I will mask the canopy prior to tacking it in place.

The blue tack sandwiched between the canopy and fuselage seals the cockpit from overspray.  Any blue tack the squeezed out I can remove using extra blue tack or use a toothpick to press back in or out of the way.

Great idea, thank you

For closed canopies, I attach before painting, masking before gluing them on.  For open I follow exactly what Route22 does.  To prevent canopies from fogging from CA, yet give more strength than white glue, I now glue on canopies with that UV laser bonding glue.