For rustproofing purposes (with Fluid Film), is it worth removing the plastic cladding that runs underneath the car to spray? It will of course be putting them back on and I'm well aware that most of the clips will break. I've done the wheel wells and that was a "b-word" and I'm not looking forward to remove the under plastics.

 

Sorry to say, but for a proper coverage of the anti-corrosion treatment that plastic needs to be removed and the actual body floor sprayed.

 

Agreed with above. You could argue that the areas that are easy to hit with Fluid Film are the areas that need it the least, since they'll be flushed out with (salty) water when you drive or (fresh) water when you go through a car wash or spray the undercarriage at home.

Most clips can be removed intact with the proper technique. The clips on my 2010 Fit come out easily with a trim removal tool. I have one with pointy tips and one with flat tips. For the common clips that have an inner rod, you use a single tip (or small flat head screwdriver) to pop the inner rod out a bit and then the trim removal tool to pull the clip. Reverse the process to re-insert the clips. Easy peasy.

 

After the large plastic panel fell off of my car, I've been driving for years without it with no issues. My car was also rustproofed yearly.