Why heavily pitted pewter?

I recall having some minor problems like this using Dow Corning Silastic 3120 for pewter casting. I heated the mould to maybe 100C -120C for an hour or 2 and this helped drive out some of the volatile constituents which helped. Bear in mind that this caused the mould to shrink maybe 5% or more depending on the length of time heating. Pewter is above the recommended temperature for the 3120 material but for limited quantities it stands up fine from my and others experience. Don't know anything about the smooth-on product, have you asked the supplier.
Joseph Rizzo wrote:

Looks like tiny gas bubbles ?
You've either got a new mould and this will disappear after a few casts, or else you're pouring too hot for the mould material. I don't know what your "pewter" alloy is, but some of the modern lead-free alloys are too hot for some silicones.
If the mould surface starts to break up, you're too hot.