Step 2: Prepping for paint
When applying wood filler always apply thin-ish layers so that there is no rot happening and plus it has a chance to dry. remember that it will sink into holes when it dries, so a second/fith coat is always needed, so dont put too much on the first time!
I took no 'during' pictures because the dust REALLY get in the camera and screws around, so I'll explain:
Try to get as little filler on the wood as possible, just in the cracks and holes. smooth it over several times with the spatula to get the air bubbles out and then let it dry. Follow the drying instructions on the tub, and leave some extra time, nothing worse than sanding wet bog and having to start again!
Between layers make sure to wipe away the dust so that it will stick. now, get your mask on and start sanding! (belt sander would be nice...)
Once all the edges are smooth and everything looks nice and box-like, do a final wash of all sides and get your painting stuff.