A thin undercoating is less of a potential problem than thick brushstrokes of acrylic. With the acrylic based gesso coated canvases the canvas becomes too absorbent for oil paints. An acrylic underpainting which is actually a block-in of the composition is less absorbent than the gesso alone. Do not worry about this. For most situations it will be fine.What problems come from painting oil on top of acrylic undercoating?
Many well known instructors say that it is okay to do an underpainting of acrylic and paint over it in oils. Frequently the underpainting is a ';grisaille'; or an under painting in values of black, gray and white and then you glaze in transparent hues of fleshtone, etc. followed up with opaques of highlight skintones for example. The naysayers of this technique say that eventually the oily topcoat will crack and peel off (acrylic expands and breathes with weather, humidity, time, etc oils don't do this so they will crack and peel). The deal is...acrylics have only been around for aobut 30 years so nobody knows what the long term result will be. I am a professional oil portrait painter. I have done ';fun'; paintings...landscapes, still lifes, etc. with the oil over acrylic method. BUT I NEVER use it for portraits since these are an archival record of an historical human being and I want the portrait to survive for centuries and not take any risks.
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