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Would it be a mistake to completely assemble dire avengers before painting -since their guns obstruct part of their chests?
It seems like they assembled before painting on the GW site but with my own assembled squad im running into difficulties.
Oh, yes. It's probably not as bad as with other models, but it's a good idea to be cautious with the glue bottle. For dryfitting, blue-tac is your friend.
Definitely. The guns obscure the chest far too much - I always leave the guns off my models until they're painted unless they're actually not in the way of anything, and the shuriken catapults certainly are unless you've positioned them really stupidly. Leave the catapults off, paint the armour plates on the chest, then glue them on.