The best way to determine whether a substrate is suitable for encaustic painting is to do "The Freezer  Test".  The freezer test should be done before you start painting on any new, unfamiliar surfaces to ensure they are compatible with encaustic.  Apply a layer or two of Wax Medium or wax paint over the substrate you are questioning and then burn it in to make sure it is properly fused to the surface of the substrate.  Allow it to cool for a couple of hours then place it in your freezer.  A day or two later, take the substrate out of your freezer and toss it on the floor.  Not just once, several times.  Give it a few extra taps and then inspect it.  Aside from the dented corners if the substrate is suitable, the paint will still be completely intact.  If you find any paint chipping off completely exposing the substrate, then it is not suitable for encaustic painting.  Why do this test?