I am about to attach some trim over some old trim to cover up a particularly ugly doorframe/wall joint. I think before I do this, it may be a good idea to fill it, since otherwise there will be a large-ish space behind my trim. What would be the best filler to use in this case? The gap is too wide for caulk in some cases, I think, but I am not sure what to use instead.
Here are some photos: