Quick answer: Oil and acrylic paintings are normally framed without glass. The paint film keeps exchanging moisture with the air, and glazing traps that moisture against the surface. So a painting on stretched canvas goes into a frame with a rabbet deep enough for the stretcher bars, open at the front. Glass comes back only for works on paper.
The rabbet is the lip cut into the back of a moulding. Depth is the whole story here. A frame built for glass and a print will not hold a canvas 20 mm thick. Supreme Pictures does custom picture framing at 7025 Tomken Rd Unit 4, near the Tomken and Derry Road intersection in northeast Mississauga.
Should an oil or acrylic painting be framed under glass?
Usually no. An oil film never fully stops moving. It exchanges moisture with the room and keeps oxidising for decades. Put glass a few millimetres in front of it and you have built a sealed pocket. The wall cools overnight, the pocket reaches dew point, and water condenses against the paint. Mould follows. So does bloom, the whitish haze inside a varnish layer.
Acrylic adds a second problem. It stays slightly soft at room temperature, so glass held against it can bond, and separating the two takes paint.
The Canadian Conservation Institute is direct about contact. Its basic care guidance for paintings states that glazing should not touch the paint surface, and that a spacer can hold it away from the image.
When glass goes on anyway
Four cases override the default.
- Works on paper. Watercolour, gouache, ink, an original print. Paper tears and stains, so it gets the standard package with a mat.
- Pastel and charcoal. Loose pigment sitting on a surface. It needs glazing and generous spacers, because contact smears it.
- A delicate piece in a busy room. A hallway, a kitchen, small children. Protection outranks air exchange.
- A flaking paint layer. Glazing keeps the flakes inside the frame. That piece also needs a conservator, because cleaning and consolidating paint is conservator work.
All four carry the same condition: the glazing must never touch the surface. A spacer is a thin bar around the inside of the rabbet that lifts glass off the art. Allow 5 mm of air, more on pastel.
How does a stretched canvas change the frame you need?
A stretched canvas is not a flat sheet. It is a fabric skin pulled over a wooden frame called a stretcher, and the bars set the depth. Standard bars measure about 19 mm. Gallery or deep profile bars run 38 mm to 50 mm. That number gets measured first.
Compare it to the frame. A rabbet cut for a print holds about 10 mm: glazing, mat, artwork, backing. Standard bars need double. Gallery bars need triple. A shallow rabbet is the most common reason a frame the right size will not close on a painting. The opening fits. The depth does not.
Two other supports behave differently. A painting on board or panel is thin and rigid, so it sits in an ordinary rabbet the way glazing would. A painting on paper is a paper framing job that happens to be an original: mat, glazing, hinge mount, backing.
| Piece | Glazing | Mat | Mounting method | Rabbet depth needed |
|---|---|---|---|---|
| Oil or acrylic on standard stretcher bars | None | None | Offset clips or mending plates | 20 mm to 25 mm |
| Oil or acrylic on deep gallery bars | None | None | Float mount on a spacer platform | 45 mm to 60 mm |
| Oil or acrylic on board or panel | None | None | Clips against a rigid backing board | 10 mm to 20 mm |
| Watercolour or gouache on paper | UV filtering glass | Single, double or triple | Hinge or dry mount to backing | About 20 mm |
| Pastel or charcoal on paper | UV filtering glass | Mat plus an extra spacer | Hinge only, no pressure | 22 mm to 28 mm |
| Canvas that needs physical protection | Glass on deep spacers | None | Float mount plus spacer stack | 55 mm and up |
What float mounting does, and what the reveal changes
Float mounting sets the canvas inside the frame with an air gap all the way around it, so the painted edges stay visible instead of vanishing under a lip. The canvas rests on a hidden platform fixed to a backing panel.
That gap is called the reveal. Between 6 mm and 10 mm reads as deliberate. Under 3 mm reads as an error, because the eye cannot tell a narrow gap from a bad fit. Past 15 mm the frame stops looking connected.
The reveal does two things. It throws a shadow line that separates art from moulding, so colour at the canvas edge reads clean. It also shows the piece is a painting on canvas. Floater frames as a product are covered in our floater frames guide.
When is UV filtering glass worth paying for?
On a work on paper, almost always. On a stretched oil the question never arises, because there is no glass.
Light damage accumulates and it does not reverse. The Canadian Conservation Institute puts it plainly: exposure to light, natural as well as artificial, causes colours to fade and paper to discolour, and it recommends glazing that filters ultraviolet light.
Ultraviolet is the fast half of it, and plain picture glass stops very little. Tru Vue rates its Museum Glass at up to 99% of UV in the 300 nm to 380 nm range, with light transmission above 97% and reflection under 1%.
We stock regular glass, non-glare glass, and glass rated at 99% UV coated. Non-glare and UV filtering are separate properties: non-glare scatters reflections, it does not filter. A watercolour opposite a west window wants both.
How do you pick moulding weight for a large canvas?
Scale first, then load.
Scale is the eye test. A 300 mm study takes a narrow profile, 20 mm to 35 mm across the face. A canvas over 900 mm on the long side usually needs 50 mm or more.
Load is physics. A framed canvas at 900 by 1200 mm in hardwood is real weight, and three things carry it.
- The moulding section. We run wood and composite mouldings in an economy line and a premium line, and the premium costs roughly two to three times the economy. More material, stiffer corners, less flex.
- The joins. A long frame lives or dies at the mitres. A wide, deep profile resists the twist that opens a corner.
- The hardware. D rings on both sides about one third down from the top, braided wire rated above the weight, and a hook that lands in a stud. Drywall alone does not hold a large framed canvas. Without a stud, use two rated anchors.
CCI adds an instruction most people get backwards: do not nail a painting into its frame, use metal mending plates that span the gap between painting and frame. A moulding with too shallow a rabbet needs a rebuild, a call we make alongside our other frame repairs.
Why a stretched canvas never gets a mat
A mat is a paper framing device with one structural job and one visual job. Structurally, the window mat is a spacer that holds paper away from the glass so the two cannot stick together as humidity cycles. Visually, it gives the eye a margin between image and moulding.
Remove the glass and the structural job disappears. A stretched canvas is also 19 mm to 50 mm thick, so 4 ply matboard laid across it would rest on the paint rather than around it.
The split is clean. Works on paper get mats, and we cut single, double and triple mats in many colours with custom window shapes. A stretched canvas gets a reveal instead.
What to bring in, and what we measure
Bring the painting. Photographs will not give us the depth, and depth decides everything.
- Bring the piece uncovered, if it is safe to move. Where the surface is fragile, tent a clean sheet over it, held off the paint by cardboard corners.
- Bring the old frame too. It tells us the rabbet you already own and whether it can be reused.
- We take four measurements. Height and width at the outer edge of the bars, the depth of the bars, and how far the image wraps around the sides.
- We check the back. Loose keys, a slack canvas, bowed bars, a stretcher racked out of square.
- We settle the mount. Offset clips for a standard canvas, a float platform for a deep or wrapped one, hinges and a mat for paper.
- You choose the moulding, then get the price in writing before anything is cut. Economy or premium line, profile width, finish, floated or lipped.
Wrapped paint is the detail people forget. A rabbet that overlaps the face by 6 mm hides 6 mm of painting on all four edges, which is why float mounting exists. Send the measurements and a photo through our quote request.
Three redirections. A digital file rather than a painting is canvas printing, a different job. Cost bands sit in our picture framing in Mississauga guide. A standard size canvas may already be covered by our ready made picture and photo frames.
Get a painting framed in Mississauga
We frame original paintings and stretched canvas for customers across Mississauga, from Malton and Heartland through Streetsville and Port Credit, plus Brampton and Etobicoke. Bring the canvas in and let us measure the bars. Full details sit on our custom picture framing service page.
Supreme Pictures
7025 Tomken Rd Unit 4, Mississauga, ON L5S 1R6
Phone: (905) 956-7400
Hours: Monday to Friday 10:30 AM to 7:00 PM; Saturday 10:30 AM to 5:00 PM; Sunday closed
Frequently asked questions
Should an oil painting be framed with glass?
Usually not. An oil paint film keeps exchanging moisture with the room air, and glass in front of it traps that moisture against the surface, which invites condensation, mould and bloom in the varnish. Glass goes on only for works on paper, pastel, mixed media or a fragile piece, and it must never touch the paint.
How deep does a frame need to be for a stretched canvas?
Measure the stretcher bars first. Standard bars sit about 19 millimetres deep, so the frame rabbet needs roughly 20 to 25 millimetres. Gallery or deep profile bars run 38 to 50 millimetres and need a rabbet of 45 millimetres or more, or a float mount. Shallow rabbets are why frames do not fit.
What is a float mounted canvas?
A float mount sets the canvas on a hidden platform inside the frame so an air gap runs all the way around it. That gap is called the reveal, usually 6 to 10 millimetres wide. It keeps the painted edges visible and casts a shadow line that separates the art from the moulding.
Can you put a mat around a canvas painting?
No. A mat is a paper framing device, and its structural job is holding paper away from glass. A stretched canvas has no glass. The canvas is also 19 to 50 millimetres thick, so matboard laid across it would rest on the paint rather than sitting around the image.
Does a painting need UV filtering glass?
A work on paper does, because light exposure fades colours permanently and the damage never reverses. UV filtering glass blocks up to 99 percent of ultraviolet in the 300 to 380 nanometre range. A stretched oil painting is framed without glass, so protection there means keeping it out of direct sunlight.
