Alkyd primer is basically an oil-based paint, albeit an alkyd oil. If one applies that stuff to a raw canvas, you can plan on having that seep through the canvas, with stains appearing on the backside of the canvas, and eventual deterioration of the canvas. Perhaps not in days, or weeks, but years.
In short, oil paint rots canvas. Alkyd primer is an oil paint. One must first apply some sort of size to the raw canvas to protect it from the ravages of oil paint (oil primer), before applying alkyd primer.
I'm not entirely certain, of course, because this is really advanced chemistry. However, Gamblin says that their alkyd primer is designed to be applied directly
onto a panel without sizing, although they recommend sizing before applying it on raw canvas. Sizing was earlier customary before applying oil ground on panel, but this isn't necessary with alkyd ground. This makes me think that sizing is unnecessary when applying alkyd ground to canvas, as well. It has to do with the fact that the linoleic acids in the alkyd resin have been neutralized
and do not attack the substrate.
This is further vindicated by what Beckers says about their Nondrop alkyd primer. They say that it is designed to be painted on any surface directly, including weave. Since it has high viscosity, it doesn't seep through the weave. This is also true of Gamblin alkyd ground, because it is a rather high viscosity formula with much pigment. They say that one needs only apply two layers, rather than three or four, which is customary with the very diluted acrylic gessos.
I think Beckers is right, because they have great expertise, having made paints since 1865, and artist grade oil paints since 1908. Conservators say that sizing with rabbit skin glue has drawbacks because it re-absorbs atmospheric moisture, which has deleterious effects on paintings. Arguably, then, it is better to prime canvases directly with alkyd ground.
However, this is a very interesting issue, and I would be glad to get more information. If alkyd primer is ideal to paint directly on both panel and canvas, then it's very important news. Many conservators remain suspicious about acrylic and vinyl as oil painting grounds.