If you use any wood or paper containing products like Masonite, hardboard, MDF, plywood etc. you will need to isolate the painting from the support surface. That can be done by simply coating the support with tome PVA or some other sealer.

I like to mount the finished pastel on a fairly rigid and thick (1/8 to 1/4 inch) support so it will be protected from the back and will lay flat. I almost always mount them before I work on the piece so it is perfectly flat at all times in the frame, but you don’t have to. You can just hang it on the support with some archival hinges of paper.

You can use just about anything as the support surface as long as the part that is touching your painting is archival in nature.

Some stuff that I have used over the years, all archival varieties.

[*]Foam Core (with at least one archival side and preferably an archival core)
[*]Mat Board
[*]Illustration Board
[*]Tempered Hardboard (sealed)
[*]ACM (Dibond or other aluminum panels)

I prefer Tempered Hardboard to MDF regardless of what the use is going to be.

I have never used oil pastels, but I am guessing dust isn’t an issue?

As for glass in contact – I have found through significant research and testing that with regular pastels, not oil pastels, having the painting directly in contact with the glass, sealed on all 4 edges so that there is no movement and no dust can get in, is by far the best method for framing a pastel.

Some of the benefits:

[*]You will have a better looking presentation, appearing as if there was nothing over the painting if you use Anti Reflective glass. Don’t use Museum UV glass for this as it has an easily scratched coating on the inside, but rather use Water White AR with or without the UV with no concerns.
[*]There is no way the painting can buckle or curl inside the frame. It will always be perfectly flat.
[*]If sealed in a relatively dry room the chance of mold / fungus growth seems to be lower than leaving an open air gap where spores and humidity can migrate inward for years to come.
[*]The amount of pastel dust that leaves the painting surface, particularly if the painting is transported long distances, is clearly less with the glass contact method. I would estimate after some bumpy road tests that the amount of pastel dust that leaves the painting can be 10 times greater with spacers.
[*]Any pastel dust that does cling to the glass (often it is almost imperceptible) stays right where it belongs, rather than rattling around in the open space. You will NEVER have dust appearing later on the inside of glass or at the bottom that is out of place and causing a distraction.
[*]Framing is significantly easier since you have a totally sealed package much like an oil painting done on a panel.
[*]No objectionable pastel dust ever visible on the spacers or on the mat since there won’t be any.
[*]The customer can change frames with ease later if desired.
[*]Shipping is a piece of cake. ZERO worries about crud being bounced around inside the air gap like with a mat or spacers. It can can even ship upside down with no concerns, eliminating the need for high dollar no tip shipping parameters.
[*]No need to ever open it up to clean the inside of the glass years later when it can become pretty trashed with airborne dust and pastel fall off.
[*]Works well for all but the most humid tropical environments, which would also give the same problems with a spacer or mat framed solution.

Some may comment on this thread that this is a very bad technique, probably having never tried it, but I use it whenever I can with fantastic results.

https://www.wetcanvas.com/forums/showthread.php?t=1298251

http://www.artistsnetwork.com/articles/art-demos-techniques/pastel-pointers-blog-passe-partout-framing

Don’t forget to ask this in the “Pastels” forum. A friendly crew and really smart as far as materials and methods.
I use acid free foam core for my pastels; at least I used to. Now unfortunately allergic to the dust.
Derek

Sorry to resurrect an old thread, but I’m wondering whether I need backing boards when framing oil paintings. Mostly I have oil on hardboard/Masonite, but I also have some oils on stretched canvas. (With the hardboard, I usually just secure the board to the frame with glazier points or such. Not as sure how to secure stretched canvas to the frame.) In either case, do I need a backing board?