I've been experimenting on and off for years with trying to cleanly layer Oil Pastels and display them like "real" painted artwork.
These new findings now let you apply/mix/blend color as long as you like, than instantly fix/isolate/dry layers and display it without glass. The only other things that I can think of that even compares would be Genesis Heat Oils or Encaustic.
The available Oil Pastel fixative sprays are not so protective, IMHO. They may prevent smudging, but the surface can still be poked or scratched. So, I've been looking more into brushed finishes.
After reading Creevy's book, I've also tried many acrylic and other mediums, sprays and gels to isolate layers - with varying results. Soft gels and matte/gloss mediums work well. While they often didn't let the underlayer smudge, they tend to be slick and fragile. They also give light oil pastel overlayers a more thin, slippery, translucent appearance.
Thin coats of Clear gesso and Golden acrylic ground for pastels give a nicer tooth to layer heavier oil pastel over, but can make the colors underneath - particularly the darks - look more dull or hazy - especially if you keep adding subsequent coats.
My latest finds:
- Layering and finishing with Holbein "Hard Gel" Acrylic Medium (which come in the nifty, new poly bag)
- Using a medium-to-fine textured sponge while layering.
The Hard Gel does just what it says, it dries to a crystal clear, flexible surface that, according to Holbein, is "hard enough to draw on" and makes a good, very low-tack, final coat.
It did the best with my ultimate "pick at it with your fingernail" test. The only thing harder and clearer would be a deeper coat of Liquitex Pouring Medium, which gives the piece that "encased in resin" look, which radically alters the final appearance of an Oil Pastel.
The Hard Gel doesn't have to go on as thickly, and isn't overly-glossy when used thinly. Because the darks in OP tend not to have much depth, (and I tend to work light-over-dark), I don't mind adding a little gloss while layering.
If the finished pieces seem too glossy after you add a final, isolation coat of Hard Gel, you can adjust it with any of the matte/satin UV varnish sprays. This will also add the recommended, removable kind of archival finish over the Hard Gel's "isolation coat," and adds UV/fading protection. It can be removed with the recommended solvent, while the Hard Gel acts as a barrier to the Oil Pastel below.
The Hard Gel also isolates Oil Pastel layers quite well. You can apply it easily with your finger to an area with just a light touch, and it dries quickly. But it can still give that "slick" surface to draw over.
Now, HERE is a new wrinkle!
If you still want something with a little "tooth" on your isolating layer (without the added haziness of acrylic ground mediums), just lightly tap a fine sponge over the gel as it dries. You can experiment with various textures and finishes this way, and the clear Hard Gel holds the tiny peaks well while still maintaining a relatively thin layer.
You can then freely overlay fresh, clean, light-over-dark oil pastel with no fear of disturbing the underlayer. The toothy texture allows for more of the medium's charming "broken color" effect that lets the under-painting colors peek through - all without smearing or mud.
Your new layers will now "POP" over the under-painting - and you can still soften edges with your finger in a wet-over-dry scumble effect, or (lightly) using a clear blending stick or tool.
And yes, believe it or not, you can now even use turps or solvent techniques on the new layer without messing up the lower layers!
The Hard Gel may be a little more resistant to scratch-back techniques, too, (but you simply can't overdo this with any acrylic coating).
Also, you may need extra isolation layers over the oilier pastel brands, heavier impasto, or added clear or blending stick layers - (if it still will work at all in extreme cases. I've yet to experiment as much with this).
I figure just as long your isolating or final layer of Hard Gel is also covering some edges or internal areas not totally covered in oil pastel,(including adhering to bits of previous gel layers) it will form a fairly tight, cohesive surface bond and can be displayed without glass (just avoid direct heat, sunlight or sharp objects and you'll be fine!).
Most of the above layering/sponging/varnishing "secrets" have also been successfully used with many of the other acrylic mediums I mentioned. So you can still try them with whatever mediums you have on hand.
But I've now found that the Holbein Hard Gel works best, so far. It's more expensive, and harder to find, but it is the usual quality you expect from Holbein. I also predict those poly bags, (also used now by Sennelier), will start to catch on with many other art materials!