What is the size of the casting you are casting and what is it's weight. What is the pouring technique bottom pouring or lip pouring?
Have you tried pouring the same casting in a different metal like WCB. What was the observation(This is to eliminate doubts regarding moulding material)
How was the porosity revealed visual inspection,machining or radiography?
Ideally we ought to know the weight of the casting, the general section thickness and overall height of section. Casting geometry would considerably affect the outcome also.
However, all things being equal !
You must ensure that before closing the mould, the paint (especially in pocketed areas) is completely dry. A 'diluted' application of zircon/isopropanol paint initailly. Flame this off and follow with a standard application of zircon/isopropanol.
Ensure that if chills are used they are completely free of any surface oxidation deposits. Also ensure complete drying around chilled areas as often paint can be drawn upsides the chill and sand interface via capilliary action.
The ingating system design is of considerable importance. 'Squirting' caused through incorrectly dimensioned ingates could cause entrainment of air within the liquid metal stream. Similarly, inadequate positioning of ingates could have the effect of producing 'confluence weld' defects - especially if insufficient superheat has been applied.
You may consider using ceramic filters at positions immediately before the entry of the ingates into the mould. This will serve to reduce any turbulence of the metal stream.
Is your liquid metal refined in a VOD or AOD vessel? If not it may be wise to be extra careful with the amount of liquid metal superheat you apply i.e. The possible concentrations of gas absorbtion into solution increase with increasing temperature.
