I did some MDF pumpkins for Halloween that were outdoors for a few days without any warping. I used spray paint.
They were smaller so that may have been a factor too.
I once asked on a different forum what people were using to seal their MDF signs against weather. The consensus was fiberglass resin (just the resin no fiberglass). But I have yet to try it out.
FYI: To save time when I do MDF signs I like to do them as a 2 part profile cut, instead of a pocket operation. The top layer profile cut out, the bottom player with a shallow profile cut so I have a alignment guide. Here is an example.