bluep92877 on "What filter would I use to alter custom field data just before saving?"

I have a custom post type I've created that has custom fields attributed to it.

I'm trying to use the wp_insert_post_data filter to modify form values.

http://codex.wordpress.org/Plugin_API/Filter_Reference/wp_insert_post_data

To help with debugging, I've done a data dump of the $data and $postarr in the callback.

Whenever I click "add new" I can see the values that have been setup. These array values however do not include any of the custom fields. When I click edit on an existing field, this hook doesn't fire. Should it not since the codex says 'when updating a post' in addition to saving it for the first time?

How do I access my custom fields as well as other fields such as the Featured Image field? Am I using the correct filter, and if not, which should I use?