I do the insides of my Brompton frames, except for the seat tube where any oily or greasy substance might interfere with the folding. On Bromptons the rear triangle chainstay bridge is prone to rusting out, at least on old models, as there is a little hole that catches water and muck coming off the tyre. The "chainstays" and "seatstays" on the triangle are open-ended, I left these open on my first Brompton, it collected a lot of debris, especially in the RH "chainstay". So on subsequent Bromptons I treated everything I could get at with my can of LPS3, a sticky rust protector dating from the 1970's, before sealing all holes and openings with window sealant. I periodically clean and repaint the inside of the seat tube, as this is another area that rusts on Bromptons that are used regularly in wet weather. And I block off the opening in the top of the Brompton seatposts (and any other open seatposts) with window sealant.
Also did my 1977 Woodrup frame with LPS3 when I had it repainted, although I couldn't see any internal rust. Not done it again in the past 35 years, and it should not need doing again as I only use this bike in good weather and it is stored in a reasonably dry garage.
Not bothered doing the frame on my old utility bike, if it rusts out I already have a replacement. Not bothered doing my wife's two (old) large-wheel bikes, as these are hardly ever used in wet conditions.
I haven't yet done my Thorns, but I will try and get some ACF-50 and add this task to their next overhaul, as it looks much easier than using my semi-liquid LPS3. The most vulnerable part on a Thorn frame with Aheadset seems to be the steerer tube/head tube, I currently grease this area periodically.