Priming Applying spray primer?

The Mr Surfacer 1000 don't have a lot of coverage. I think there are other versions of primer in the same line with different coverage, but I am a bit unsure. I used the Mr Surfacer 1000 myself for one project because it was a head that was to be sent off to a casting company and it was one of their preferred primers, but I honestly didn't like it much. It was also very expensive compared to what I normaly use.
I did send the a sample of one of my usual primers when I sent the head and they tested it and said it was fine, for future reference. If you are using a casting company instead of doing your own casting, then you could always ask if they could do the same. I had still tested it myself first though, to be polite and not waste their time with something I did not believe in myself.

I do most of my casting myself now and I have tried many different primers. Mostly local brands and I don't know what type of silicone you will be using, so names will probably not be of much use to you.
But this is my method for testing a new primer:
I spray some on a neutral surface or on a scrap piece of the sculpting material, making sure to cover it well. I check to see how long it takes for it to dry and then check to make sure it has not reacted with the sculpting material in any way.
Then I mix some silicone of the type I intend to make the molds out of and either make a simlpe mold or just dribble some on, depending on the object. I may dribble some from the same batch on a surfance I know for sure to work with it, just as a control to make sure the silicone was properly mixed.
I demold the silicone and check carefully to make sure that is has cured well and that nothing funny is going on with the primed surface. If I make a control piece I will compare them carefully.
Then, just to be on the safe side I will test cast some of the resin I am going to use in the mold, making sure to get it on the surface of the silicone that was in contact with the primer.
If all is well it's likely safe to use on the doll parts.

Most of what I've used so far are different typer of primes intended to prepare a steel or plastic surface for painting it with spraypaints, such as for cars and other outdoor uses. These primers are usually a neutral grey in colour (designed to show imperfections and small details very well) and cover pretty well, although different types will behave a bit differently. They also sand very well.
I would advice against using gesso on anything you intend to cast. It is a viable option for one off artist dolls, though, but then you wouldn't really need the primer.

 

It hat case, if you need a primer I would suggest you try to find one that is ment to prime the surface for the type of paint or gesso you will be using (primer for acrylic paint if you use acrylic paint or gesso, and so on).
That's what I did for the ork doll I'm making and it seems o be working well.