The same question appears repeatedly on various social media sites that utilize crowd-sourced responses. The question: “Is it alright if I see some oil coming through to the rear of my painting when paint is applied to the surface of my stretched canvas?
The simple answer is, “No.” But that would cheat you of an explanation and also be the shortest essay I have ever written. When paintings began to transition from wood panels to stretched canvas a number of changes needed to occur. While oil penetration does not aggressively attack the integrity of a wood panel, the same oil infiltrating canvas is a whole other story.
When the advent of liturgical banners for processions started to appear, the smart choice of substrate for banners was canvas. Images of Jesus, Mary, the saints or apostles would be carried on poles in the streets as part of a church celebration. A large wooden panel held high would be both cumbersome and could likely result in injury to someone in a procession if a stiff wind toppled a solid wood panel. I doubt that martyrdom would extend to being killed by a falling panel containing the image of a saint. The smart choice that also allowed the image to be large and seen from a distance was realized in the use of painted canvas.
Painting on canvas for banners evolved to static displays. Stretched, primed and painted canvas provided an artist with a large surface area minus a tremendous amount of weight if a commissioned painting needed to be large. Canvas could be woven and stitched together to create paintings in grand sizes limited only by where a painting would be hung. A loom could produce canvas of nearly unlimited continuous yardage. (Want to experience a really large painting? Musee du Louvre: Jacques-Louis David, The Coronation of Napoleon, 20 ft. 4 inches by 32 ft. 1 inch - 6.21 m x 9.79 m.)
Next, formulations for grounds evolved over time. I will save a discussion of traditional/historic formulation of ground preparations for another time. Needless to say, artists understood the need to isolate the canvas from the paint that would be applied. Grounds also provided a means to smooth the surface of a painting by filling in the weave of the canvas and create a uniform tonal color to complement and unify the elements in a painting. This was especially important if an artist employed a rough woven fabric, like burlap, that was both loosely woven and contained a significant number of slubs (thick spots in the thread) that would be visually noticeable and potentially distracting.
Artists used practical, natural materials that were available. Animal glue, chalk, gypsum, pigment mixed with a glue binder, all played a role in surface preparation. Realize that acrylic priming formulations would on appear until as late as 1955-56, a mere blink of the eye in relation to the long history of canvas paintings primed with earth and animal materials.
Basically, the glue and mineral combinations would provide a fairly good method for isolating the deleterious effects of the oil binder in paints. As we know, drying oils oxidize as they dry. Recall from your science courses that oxidation can be both rapid and slow. Fire is an example of rapid oxidation. A drying oil oxidizing is a good example of the same chemical process occurring very slowly.
Canvas oxidizes over time as well. It become more brittle. Oil paint oxidizing, as it imbibes oxygen, also become brittle. So much so that most paintings hanging in museums today have at some point been stabilized by lining them from behind with a modern canvas or synthetic woven material to buttress a brittle, weak original painting support. Early conservation treatments used wax as an adhesive to adhere a new canvas to an old one. It was once considered a standard practice of care for paintings.
The process involves heat and pressure and an unfortunate consequence of this lining method has contributed to flattening some of the impasto and has also resulted in having the wax show through within spots the image to a small degree when the wax infiltrated any small voids in the painting support. Wax lined painting do not stand out glaringly and are mostly noted only by conservators with a trained eye that detect the tell-tail signs of a wax lining treatment. Regardless, these paintings are still wonderful, well cared for and will live long into the future in museums throughout the world. The consequence of not treating them and letting brittle paint and canvas deteriorate would be inexcusable.
The infiltration of wax into the viewing surface of a painting is a good segue into addressing the original question. Seeing oil penetrating into the back of the canvas is a sign that the isolation/priming process is flawed. Either not enough layers of sizing or ground/primer have been applied or the wrong material is being used.
While a few traditionalists still employ animal glue as the first coating for a canvas, that material has proven to cause accelerated embrittlement and the glue reacts to humidity by swelling and shrinking. Given enough expansion and contraction cycles, the ever-stiffening paint layer applied on top of the ground layers will react to the tension imposed on the paint by cracking. The energy has nowhere to go so the paint breaks to relieve the stress.
Modern priming methods and new thoughts on surface preparation have provided vast improvements to increase the longevity of a canvas painting. We have documentation that solid supports are wonderful but, in many cases, very large paintings will still need to be constructed using stretched fabric because the weight of a solid support might be impractical. The stability of canvas paintings can be aided by attaching a backing board to the stretcher or strainer on the rear of the painting.
Now artists can apply synthetic sizing like PVA to a raw canvas followed by two or more coats of oil or acrylic ground. Manufacturers of these products provide helpful instructions on surface preparation.
However, for artists who have been priming their own raw canvas for many years, I suggest they perform a simple test to assure themselves that they are mitigating against oil penetration.
Prepare a small raw stretched canvas (8 x 10 or 11 x 14) and divide the surface into 2 rows with 3 squares in each row for a total of six distinct squares. Use the canvas brand that you are accustomed to purchasing. In the first square, apply the method you have been traditionally using to prepare your canvas for a painting. In each of the following squares, add more layers of sizing first and then add more layers of primer. Note carefully what you did on a separate sheet of paper. (Example: Sq. 1: 1 coat of sizing, 2 coats of primer, Sq. 2: 2 coats of sizing, 3 coats of primer, Sq. 3: 2 coats of sizing, 3 coats of primer, etc.)
Next, lay your stretched canvas flat and let it dry for at least 1 one month. Add more time if you prime using oil ground and/or you put 5 or 6 coats of primer on your last two squares. Last, using an eye dropper or something similar, create a rather generous circular puddle of drying oil into the center of each square. Keep the canvas laying flat. Turn it over after 3 weeks of drying. See if any of the oil has permeated through to the back. Check it again a few months later for oil penetration. Adjust your priming technique if your original method fails to stop oil penetration. Use the method that shows no oil infiltration.
Your grandchildren or a collector will be thankful that a painting you did using a sound method of priming will not need to be treated at considerable expense to them.
The Syntax of Color