There are no silly questions we all learn from these posts.
Underseal can hide a myriad of issues. I use therefore clear waxoyl.
But as your chassis is undersealed already I would repair it by checking the galvanising is ok. If not repaint it with cold galvanising paint after rust treatment & then reseal it.
You should also oil the sealing rubber that are between the wings & the body. You will find posts on here about that.
