All Purpose Filler is very versatile stuff - it says in the name, doesn't it? I've been using it to create all sorts of surface textures around my forest. I found a kilo and a half box of the stuff hidden in the attic a few months ago and although it was a bit clotted, I figured it was worth trying it out. My standard practice is to mix up an acrylic base colour into the wet plaster. A colour that more or less suits the purpose. For example, brown for soil, grey for rocks and roads etc. It will never be any use as a surface colour because the stuff dries much much paler than the original mix and without getting into scientific part for part calculations, the final colour is a bit hit and miss.
So why do it? Because the shooting process is extraordinarily violent as far as the set is concerned: equipment cracks and bashes and scrapes it's way around the set and then drills gouge holes for fixing puppets down. Then glue and tack rips up paintwork where props have been held in place and sharp things get left lying about all over the place that scrape and knock and drag.... If the beauty of the rendering were only skin deep, there would be all sorts of blinding white scuff marks all over the set by the end of the first day! By colouring the base plaster, any chips expose a fragment of suitable colour instead and the damage is, for most practical purposes, invisible. Clever huh? I wish I'd thought of it myself but I must credit Christopher Payne in his awesome book "The Encyclopedia of Modelmaking Techniques" Required reading as far as I'm concerned.


So the next move is to wash the whole set with diluted PVA glue. What for? This is from my experience early in my career, building life-scale sets. The intensely porous surface of the rendering will soak up more paint that you can possibly imagine making it very difficult to get a consistent finish. The traditional way is to boil up horses hooves to make a gelatine size that you slap on with a big smelly brush. That's not done any more. Thank goodness for science.
At this point in the process it's hard to believe that I've made any progress at all. Nothing seems to move on from day to day. The trouble is, it's a waiting game. A couple of hours work with runny plaster or PVA wash, or wood glue and I have to hang about another day for the work to dry in order to do the next thing. It's why I have time to write this. Looking through pictures - I've taken hundreds, you know - I can begin to see that actually there is some development taking place. It's a bit like watching a child grow. Nothing seems to change for years and suddenly he's fourteen. Who saw that coming?