I mean theoretically WD40 would protect your instrument from rust. I’m not sure what kind of affect it would have on the sustain or resonance. I’m also not sure if it would affect the glue.
But realistically, WD40 isn’t something I’d be wanting to be touching with my hands all the time. It even provides on the back of the bottle a caution to avoid skin contact. I think if even WD40 recommends that it is not safe for skin contact then I would be very hesitant to be putting it into my handpan. The smell is also not very pleasant.
But it comes down to your preference and beliefs.
I think there are far better products out there. This is why I recommend the products that are tried and tested. Phoenix oil and Pure Sound Wax are certainly my go to products as they have been specifically designed for these instruments with the safety of the players in mind by using natural and plant based products.
It may seem like it’s an expensive purchase for a Phoenix oil bottle for example, however consider that you use about 5 ml each time you clean your instrument. If you clean once a month for example, your AUD$35 100ml bottle will last you nearly 2 years!
Hope this helps. Again, my website has a lot of information as well as a little video to show how to apply these products.