General Cabrio Mk1 Sill Treatment

[77jm]

I want to spray the insides of my Cabrio sills with waxoyl or similar. Where do I drill to get the best coverage?

I was having a prod around and noticed that the underseal was crunching when prodded so I believe it is going from the inside out.

It passed an MOT 2 weeks ago with no advisories so can't be bad.