That's one heck of a Hacienda!! :oI've never tried polyfilla on foamcore is there any risk of warping or cracking as it drys?
Not that I have experienced so far. I don't load too much Polyfilla. Its just enough to skim the surface. I then dip the palette knife in water and smooth over the render. This helps to take some of the roughness off.When its dry it gets a rub down with sandpaper.
So far no cracks and no warping. I use 5mm Foamcore, so its pretty thick and rigid to start with. Its also braced inside, so I am sure that helps to keep it rigid. If you look at the church model I made. There are some long expanses of quite high walls and they show no sign of warping. So far so good.
