Yes, you can definitely incorporate collage material into your cold wax/oil paintings. I sometimes use Gamblin's PVA size to coat the front and back of my papers - but this is optional, according to
Gamblin Colors.
Here are questions I ask myself to determine whether or not a collaged paper element should be pre-treated with PVA Size or not:
Is the paper integral to the structural integrity of the painting? If “yes”, apply PVA Size first. This is always the case when the paper is the primary support on which the Cold Wax/Oil Color and other collage elements is applied.
Will it be undesirable if the collaged paper changes in appearance or deteriorates over time from the acidity of the Cold Wax/Oil painting? If “yes”, apply PVA Size. Examples might be a decorative paper that is not intended to “disappear” into the painting or maybe a photograph that needs to appear as unchanged over time as possible.
In most other cases, we want the paper to absorb the wax and oil so it becomes less visible and establishes a continuity of material between layers. Whatever imagery or material is applied to the collage paper comes forward while the paper itself takes on diminished importance.
Here is the official response from Gamblin Colors regarding Collage material - paper or fabric - used with cold wax/oil:
Below are a few summary points about collaging into oil paintings. To this I would add- when Cold Wax Medium is included in the oil painting process, use a similar amount of Cold Wax in the collage medium as was used in the paint layers beneath. Like materials bond together best, so adding wax into the medium used for collaging will improve the bond between layers.
Collaging Paper or Fabric into an Oil Painting
Paper and fabric may be added as collage elements to an oil painting (or directly on Oil Ground before painting) using a thin coat of Galkyd or Galkyd Gel. Galkyd is a fast-drying viscous fluid medium, while Galkyd Gel has more body and dries a little more slowly. In either case, it may be necessary to apply the medium first and allow it to become tacky before adhering the collage element in place.
Rigid Supports
When using collage elements and oil painting together, consider working on rigid supports (wooden panels) rather than stretched canvas. Eliminating flex from a collaged oil painting will improve the long-term stability of mixed media oil paintings.
Protect Collage Elements with PVA Size
To protect collage elements from the free acids present in oil colors and oil mediums, the paper or fabric may be treated with Gamblin PVA Size front and back. Make certain the PVA Size is completely dry before adding collage pieces to an oil painting. While a thin coating of PVA Size will help the collage piece to retain its original appearance, it will also reduce its absorbency. This reduced absorbency may affect how well the collage element bonds to and between oil paint layers.
Collaging Without PVA Size
If there is less concern about the potential visual effects of an oil medium used to adhere and glaze or paint over paper or fabric, collage elements may be used without PVA Size. This may be preferred in some instances- like when the objective is to have a thin paper or fabric “disappear” into paint layers. (e.g. an ink drawing on thin kozo parchment paper) Collage pieces may actually bond better to and between oil paint layers without PVA Size. Still, remember that adhering and glazing collaged paper or fabric with Galkyd will most certainly result in some discoloration of the article over time.