You don't need protection inside the frames if they're airtight, i.e. seatpost greased etc. Rusting cannot take place without a lot of oxygen
WD40 is also a good "Bolt loosener".
That stuff and bikes don't mix.
If the bike is stored, how can the bolts loosen? Does WD40 have stored energy?? Naw! It was developed to prevent intercontinental missiles in their silos from rusting. It also does it for intercontinenal bicycles.
Try not to mothball your bike for the Winter, or it just won't get used! Spray the insides of the frame tubes with WD40 or ACF 50 next time you are doing any maintenance. Put a very light film on any exposed steel work in late Autumn, keep the chain nice and oily and just take the bike out for a ride any time you can during the dry days in Winter (the more the better) . In no time at all Spring will arrive and your bike will still be in good shape for riding. This has worked for me for many, many years and many bikes, and I've never suffered from corrosion, even with the ones that had to be parked outside.
You can buy spray on or brush apply , a light greased based material from agriculture merchants, This is used to protect steel surfaces when not in use, like plough-shares. Sounds a bit extreme, but it works. Try your nearest John Deere tractor agency.
