I have many HO scale 1/2', 85' pipes. I want to paint the insides a sort of rustic color, then paint the outside a kind of greenish coated color. I want to use craft type (cheap) paints for this. I am thinking maybe a cotton ball attached to wire for the inside covering. Any thoughts or suggestions are most welcome.
That IS a puzzler. I'd first check to see how far into each pipe I'd likely see when they were in place on the layout. I'd then mix up a light paint wash of the interior color and dip the ends into it as far as needed. Then, I'd wipe off the color from the exterior and let dry. You'd get good interior coverage without much fuss. You might need multiple coats. This sounds like a neat project.
I would take a board and drill a bunch of 1/2" holes in it. Then stick the tubes in the holes and use a can of spray paint or air brush to spray down into the tubes. Let it dry, then turn it over and spray from the other end. I wouldn't worry too much about getting all the way through. Then get some corks or telescoping tubing, put it in the end of the tubes and spray the outside the of tubes the green color (removing the stopper/tubes as soon as possible so they don't get painted in there.) If you run a cotton ball through the paint will be very uneven and smeary.
If the ID of the pipe is 1/2" then I'd get a piece of sponge, the natural stuff works best, about 3/4" in dia. and hot-glue it to the end of a 1/4" wooden dowel. Thin the cheap acrylic paint a bit run the pipe over the dowel then sparingly apply the paint to the sponge and pull the sponge back through the pipe. Allow the paint to dry completely then repeat until you get the desired level of paint coverage. Thoroughly clean the sponge between uses. Alternate between ends of pipe to keep the paint even.
Plug one end, pour in (maybe a little thinned...) paint of your choice, stop up the other end, shake vigorously for five seconds, and remove the stoppers, one end at a time. As you unstop each end, set that end on some wax paper or something only semi-absorbent, wait-two-three, and then unstop the other end and place that end onto the same surface, not the same place. Wait-two-three, and then hold it up and have a look with some light. You might have to touch up the inside ends a bit, but don't do that until the original stuff is dry. Probably several hours to be safe.
Wow! Some really great suggestions.. I gotta consider what has been offered. These are intended to be loads of gas type pipe loads, that have a greenish outer coating. Thanks guys for the opinions, they are all great.