Bubbles... Tiny bubbles

I've used straight latex paint, of an appropriate colour, brushed on rather vigorously.  You really need to work at getting it to fill the pin holes. (painters term for what you have there) The nice thing about having to really work the paint in is that you will work it out of the mortar lines. If you concentrate mostly working side to side, your brush will wipe the paint out of the, mostly horizontal, mortar lines and tend to force it into the holes.

Also, depending on what colours you want the final piece to be, the more speckled you make the rock colour the more the pin holes will blend in and disappear.

Bubbles are always an issue when dealing with castings. While your example certainly has lots of small ones, it's definitely not as bad as it could be.

I'm, not sure that holes will

I'm, not sure that holes will show up in mold , but I suppose tear our could be an issue .

I think I would just dry brush something on the surface , like maybe carpenters glue ... The problem is of course the holes are so small that water surface tension will prevent anything from going in , unless you add detergent .

Any oil based product you run the risk of it containing solvents ,which will disolve the surface and expose the cell structure ...holes .I assume that its a foam like urethane or that it had a lot of out gassing in the curing.

what about...

I've no experience with making molds, but you're going to create a negative and then cast from that, right?

So why not just make the negative? Either the casting material will go in the holes or it won't. If it doesn't, then you're home free. If it does they might tear away when you separate the two, also fine. If the material goes in the holes and pulls out of them clean, leaving bumps on the negative, you can clip them off easier than you can fill them in the original, right? Like with a sprue cutter or something?

Progress so far

I am trying the thick latex paint, but it does not seem to work so far.  Also trying Aleene's Tacky Glue and that shows better promise. Worried that both may obscure some of the mortar lines.  Will post pictures when they dry. 

As for the actual mold itself, I'm leery of taking a chance with holes.  One is suppose to spray a mold release before pouring the mold material.  Looking online and actually talking to the mold manufacturers, spraying PAM is almost as good.  Maybe some of the oil will get into the holes and prevent the mold material from entering.  But again, I'd be taking a chance.

If thick latex paint is not

If thick latex paint is not going in then I don't think your going to have a problem , The mold compounds I have used in the past are just latex rubber and its even thicker.

I quit

The paint and glue did not work out at all.  So I then made a quick and sloppy mold of part of the tunnel.  Then cast it with plaster.  Every hole showed up.

I'm not sure where I got this portal.  I think it may have been a copy, because of all the holes and the poor detail.  I'm just going to buy a new one.