I have a surface made of wood (teak to be exact) I need to take a mould from it. However, I CANNOT damage the original. That means no sealers, primers, fillers, waxes etc. Will PVA alone be a good choice for this? The surface looks quite porous...

Over finished old wood I used 4 coats of wax with good time between layers of wax so they can adequately dry and not just wipe off previous coat, then spray a light mist of pva over that with automotive spray gun. Leaving the wax to sit between coats for a good length of time has been key for me in protecting old surfaces. Of course make sure you are buffing lightly to a shine after each coat. I use filleting wax for any edges that need it because I can shape it with custom tools and a heat gun easily.  Mold should pop out nicely.