On the trailer tongue extension I wouldn't use paint in any case. Marine grease will seep deeply into the pores of the metal much better than paint will. Cleaning out the inside tube the best you can is a great idea, I bet there is a lot of rust in there. Some type of spray that breaks down rust would be the best thing. Use an electrician's wire fish to pass a string through the tube, the tie a couple bunched up rags to the string. Soak the inside of the tube and rags with a rust penetrant. Pull the rags through... You'll probably have to spray and soak then let sit, the do a few few pulls with rags, spray and let sit and repeat... It will be annoying and take a while but it will probably be as clan as it will get. Get a good thick coating of grease on the inside... Wire brush and steel wool the extension, the rust spray will help clean up the fine rust when you get the big stuff off. I wouldn't worry about getting it to clean, shiny metal... just go 95%. Get the most 'sticky' marine breast you can find and brush it on with a chip brush, don't be shy.
The reason extendable tongues rust and stick is because people don't do annual maintenance! Before laying up for the winter pull it and give a thick coat of grease.
BTW, paint is not water proof... Water molecules eventually work their way under the paint and you end up with bubbling rust. Grease will really work its way into the metal and keep it from rusting as long as you keep up with the grease!