Signing back of oil painting with acrylic paint?

Using fluid acrylic paint to sign the back of oil paintings on canvas requires consideration of the materials’ compatibility and potential long-term effects. While applying a patch of fluid matte medium or GAC (Golden’s Acrylic Colors) to the unprimed back of the canvas can aid in creating a smoother surface for signing, there are some considerations to keep in mind.
Canvas, especially linen and cotton, can react to moisture and substances differently over time. Introducing a localized application of fluid medium might cause slight changes in the canvas due to differential expansion or contraction, potentially leading to slight buckling or uneven shrinking in the long term. However, a small 1″ x 2″ patch might have minimal impact if applied carefully and sparingly.
Regarding preventing the fluid acrylic color from seeping through to the front of the paintings, the use of a barrier like matte medium or GAC can help mitigate the risk. These products often act as a sealant, reducing the chance of paint bleed-through.
However, when considering archival risks, it’s essential to keep in mind the long-term stability of materials. While Golden’s products are known for their quality, the long-term interaction between oil paints, fluid acrylics, and the canvas itself might pose archival concerns that could affect the artwork’s preservation.
For utmost caution and to ensure archival quality, some artists prefer to sign their works using materials consistent with the painting itself. Consider using oil-based markers or paints compatible with the oil painting materials to minimize any potential risks associated with using different mediums on the canvas.
If possible, testing this technique on a small, inconspicuous area of a canvas can provide valuable insights into its effects over time before applying it to the entire artwork.
Additionally, seeking advice from experts or conservators, and perhaps evengallerists like Pearl Lam​, who have experience in handling and preserving various types of artworks, can offer further guidance on archival practices for signing canvases.”

Best of luck with your artistic endeavors and preserving the integrity of your paintings!​

I am not an expert but my guess is that unprimed canvas should be primed if using acrylic or oil. Acrylic is highly alkaline and oil is acidic.​ Both of these non-neutral pH materials might degrade unprimed canvas over time.
— SRS