This may be a dumb question-should you seal a model with dullcoat after you’ve weathered it with chalks? Also what about after using artist oil washes[?]
You need to seal it with dullcoat or the chalks will rub off when you handle the plane (like in zooming it around your house:) .
Unfortunately I can not help you with the oil wahses, I 've never used them.
You must seal after using pastels and should after oil washes. Oils don’t really ever dry.