Wait just dog gone, cotton picking minute here...
Am I the only one in here thinking that if they are having such a problem with spatter on the joints, this tells me point blank that your welding parameters aren't even close to being set to produce optimal results - plain and simple... You either have too much wire feed speed and not enough voltage, or your voltage is set right in, or near the transition zone and your method of transfer is globular... I would suggest that you post your settings for your complete welding parameters and maybe we can get to the bottom of this because that sounds more like a process problem... I mean, I know I'm just speculating here but, for this project, are you using a pre-qualified WPS or did you go per section 4 and qualify it yourselves? When you tested the weldable primer (If you did?), were the welding parameters the same as what you're using now?
I ask this because if you qualified the WPS with everything included - especially the weldable primer, then you just need to duplicate those optimal conditions in order to fine tune your settings so that there's only a fraction of the amount of spatter you're currently experiencing... Is this shop work or is the welding being done in the field? Are the power sources checked and the input power in good working order and pumping out the necessary voltage and current to run your power sources correctly? Did you double and triple check your shielding gas delivery circuits (source of gas, quality, hoses, fittings, etc.) completely? What are the condition of the guns/torches and are their CTWD's where they should be? Are the wire feeders working properly and are the conduits inside the gun cables changed regularly, and are the gun/torch cables as straight as possible so as not to impede a consistent feed rate to the gun/torchs? What are your base metals, and their thicknesses? Have you tried GMAW-P? I'll stop here, but you get what I'm asking-Details.
I personally think that weldable primer is questionable @ best... I may be reaching for straws here by writing this but, it smells like a process issue to me and yet much more details are required in order to visualize your situation with better focus than what's currently available... Just my 2 scheckels worth.
Respectfully,
Henry