I'm frequently tasked with working on projects that need fleshing out in order to become viable products. For instance, when creating reports for a specific cost analysis, changes are usually needed in order for the end result to be usable in specific meetings with key people throughout the business.
I have this CFO who loves my reports, and honestly believes I can do anything and everything with our software. Although I usually can do a lot of the items he wants in a relatively short period of time, there are cases in which he decides that the completed project isn't completely to his liking, and changes it at a whim. This is definitely not isolated to my company, but I can honestly say I haven't had a complete scope change on a report before I met him. Regardless, that's not really the issue here.
The issue comes when he tacks on extra features, and I have other projects that need to be worked on as well. For example, I will be working on a report for one of our managers, and he will ask for another feature added to the report I just finished for him, because he had the time to go over it. His changes are substantial and usually induce long periods of time solving and scaling the problem for future use.
To summarize, he's a notorious scope creep, but obviously (being my boss' boss) I cannot tell him "Sorry, I'm working on something else right now, I can get to your item when I'm finished" because his reports are usually a higher priority. It's not that he's unhappy with the finished product, he just usually sees it and feels that he could add something new to make it even better. This always results in my boss being upset that I didn't finish that manager's task, and due to this, projects tend to pile up. I have a to-do list that both my boss and the CFO can check, so it's not like the information is foreign to either of them.
I'm looking for a strictly IPS solution, and specifically I'm looking for how to deal with his requests and how to not just say "I'm busy, go away" sort of dealie. To escalate this issue (which would be the Workplace.SE answer), I'd need to go to the CEO, and although I know that everyone in executive management has spoken about this, the CEO is also guilty of it. I don't really know how to tell him that there's other projects I need to work on before I can make his changes and am really looking to speak with him about it in a way he not only understands, but that doesn't run the risk of me sounding rude or dismissive of his project.
How can I tactfully talk to a CFO about preventing scope creeping from someone in an executive position without losing my job?
Note: I understand there's a lack of communication in management, but that is not something I can easily fix.