In my humble opinion the faying surfaces are those surfaces in direct contact when the connection is assembled or put together. Whether it is a welded or bolted connection is not material to the question of what constitutes a faying surface.

As such, the slip critical connection is not to be painted where ever metal to metal contact is made, i.e., between plies of metal. 

If the steel is hot dip galvanized, the surfaces that are in direct contact, i.e., between plies, are to be roughened with a hand brush.

Slip critical used to be known as friction connections because the friction developed between the two components, i.e., the faying surfaces, was dependent on the clamping force of the bolts times the coefficient of friction of the two metallic surfaces. Paint acts like a lubricant when subjected to high clamping forces effectively reducing the friction of the two faying surfaces.

This is in contrast to a bearing connection where the strength of the connection is a function of the cross section of the bolts passing through the slip plane (faying plane) of the connection. A bearing connection can slip when loaded. As the connection slips, the body of the bolt fetches up on the sides of the holes and is subject to shear forces.

A slip critical connection should not slip or slide when subject to a load. Think of it this way, in a bearing connection the bolts can be replaced with pins. The capacity of the connection is dependent on the cross sectional area of the bolts (pins). In a slip critical connection he bolts can be replaced with heavy C-clamps. The load capacity of the slip critical connection is a function of the clamping force times the coefficient of friction. The greater the coefficient of friction, the greater the capacity of the slip critical connection. Anything that reduces the coefficient of friction reduces the capacity of the connection. Oil, grease, or paint does just that, it reduces the capacity of the connection by reducing the coefficient of fiction. Thus the requirement that the faying surfaces must be free of any material that would reduce the coefficient of friction.   

When a slip critical connection is over loaded, the connection does slip and the bolt goes into bearing. The allowable load per bolt is lower for a slip critical connection than for a bearing connection.

Connections subjected to cyclic loads are required to be slip critical so the connection does not slip in one direction and then slip in the opposite direction as the load reverses. Where that to happen repeatedly, the bolt holes could become elongated and the slippage would worsen over time. Bad, bad, bad.

So, no paint between plies that are clamped together, i.e., no painted faying surfaces.

Sorry for the delayed response. I don't usually frequent this area.

Al