My usual method is to paint the cockpit parts etc while they are still on the sprue.
Once they are assembled then I'll give them a wash and weather them and this blends everything together.
The airframe is sprayed as a whole, although I might attach a cowling temporarily to ensure the upper/lower camo demarcation line matches and then remove it to make painting the cowling band easier.
Wheel wells I paint first and then stuff them with wet tissue paper to mask them off.
BIts and pieces such as wheels, undercarriage doors and legs, pitot heads, radio masts etc I paint seperately and leave until last.
Regarding canopies.
If the canopy frames are a different colour to the airframe I paint them seperately. If however they are the same colour, eg an olive drab P51 or P47, I'll attach them temporarily and spray/fade them as part of the main paint job to ensure that they match it.
Hope this helps.
Nige

I use small amounts of white glue for that , just enough to hold the part in place . once the paint has dried and any other coats of dull coat . I run it under water and this desolves the white glue . Then i finish the air frame . Hope this helps you out .