Priming Priming+sanding - when is it enough?

Well, it depends.
What type of primer are you using? Is it a sanding base type that is intended to fill in scratches and such or is it more of a finishing spray?
Generally speaking, it's a good idea to get as close to perfection as you can with every step before moving on. So if your sculpt still needs smoothing out, then it's too early for primer. How big a deal this is, however, will vary depending on what materials you are using. For example, if you are working in air dry clay, you could try to remove the primer from the low spots and then fill them in with clay, sand smooth and re-apply primer, while if you are working in polymer clay you could not do this without removing all the primer, as it's usually not a good idea to bake the primer. You could still use epoxy putty for last minute fixes, but it's a lot harder to sand than polymer clay, so it's very difficult to get the transition smooth.

Usually I spray one or two layers of primer after I have finished a piece and only buff the surface to prepare it for the mold, but of course sometimes I don't see things I should have fixed until after the primer is on and then I just do my best depending on the cirumstances.
In severe cases I make a junk cast in polyurethane and make the final fixes there.

 

Matte primers are much better than everything else, because they have the fillers in it. And even using the matte primer it's sometimes hard to achieve the fine matte surface: just a bit oversprayed - and you have satin look.

Matte finish has to be achieved with primer and not with the sanding, that I know for sure, as sanding inevitably leaves a non-uniform scratches, even if you use 1200 grit or so.

You don't have to make everything super smooth before applying your first level of primer, as one of the main purposes of primer is to reveal any issues. The grey color is the best for that. And you can apply as many layers of air-dry clay and primer on top of each other.

Also, primer is used as a filler for tiny scratches too. The bigger holes should be filled with the clay though. Previously I used only car aerosols which spray amount was hard to control, also too big areas were sprayed at a time. So losing details was a huge issue. I tried Mr. Surfacer primers, and their biggest advantage is that they can be applied with a brush. So if you see a tiny dent put a drop of Mr. Surfacer there, wait for it to dry, apply some more if needed, then after the dent is filled sand that spot.

The car primer is still wonderful for a big plains: arms, legs, body, headcap and so on.

Nobody could say how many layers of primer and how many sanding sessions do you need. It highly depends on your air dry clay brand, on your ability to sculpt smoothly, and so on. There can be MANY. I remember it took me a month to sand and paint/prime my OOAK full BJD whose height was 60 cm. And I still can find some imperfections - but I don't plan to cast him anyway.

Attention to details is the signature of your professionalism. I've seen indie head sculpts that were all bumpy and lumpy, just because the author did the head and never expected that it would take so much time to even out the surface. Like, sculpting was fun, and the tedious polishing wasn't. I'm very picky about my sculpts, especially the face part, because your customer won't be bothered with a tiny scratch on the leg, but every imperfection on the face affects faceup greatly. Well, as you're a faceup artist you're already well aware of that issue.

 

Out of curiosity, is using Sandable Gesso a good or bad move for smoothing a sculpt? I've used it a lot before for smoothing out worbla and coating foam for costume stuff.

 

Yes, especially if it's the brush-on gesso so you can control the layer thickness and amount applied. All primers I use are sandable, it's one of their most important qualities.

 

@AirimirOfGondor, you're welcome!
I believe you don't need to remove all of your primer - just try to sand it with a rough sandpaper, like 400 grit or less, or you can spray a layer of matte primer over it. What air dry clay are you using, BTW?