after plasterboarding over studded walls or dot and dabbing im finding there seems to be a slight ridge and sometimes a hairline crack where the joints are which stands out when the light shines across the wall i sand them down but just wondering whats causing it i scrim as normal and 2 coat the wall would it be better to fill the joint first leave for a while and then scrim and perhaps feather over the joint leave for a while and then 2 coat