Getting the frame painted inside is a really really messy job.
but the results are outstanding....
So I use the Rust Bullet
http://www.rustbullet.com/ to encapsulate any rust on the frame. While this frame was sand blasted the inside was not and on top of that every single seam on the frame will have rust sandwiched inside the seam. So I apply the RB inside the frame via the holes in the frame. I thin the paint down, use a honey type bottle to squirt the paint into the frame and then I use a homemade brush to get everything coated in there. The homemade brush is simply a wire coat hanger cut in half with a clean rag attached to it. I then stuff the rag on the wire inside the frame rails via the holes and spend lot's of time moving it back and forth plus I twist it a a lot. Basically everything I can do to get the RB coated on the whole inside. I also flip the frame over a few times to do this. I then go over the outside of the frame with the RB using a brush to jam and force it into every single seam on the frame, I even hit the welds with it too. I then let it cure with one end of the frame lower than the other so excess paint drains out. Once that is done it is sanding time to clean off all excess paint that has oozed out of the frame or that I spread on seams. I start with 36grit on the pneumatic sander then move to 80g and work my way up to 200g. Also takes a lot of hand sanding to get into all the nooks and crannies. In other words, lot's of hours sanding. In the following pictures I am only up to 80g and have lot's of hand sanding left to do. The RB also ends up acting like a filler or sealer since it is left in any depressions and the like on the frame. For this frame I used 3/4 of a gallon of RB.
flipping the frame over is easy with the hoist
Here are all the paint products I am using on the frame/chassis. In addition in this pile is a fuel tank resto kit I will be using on the fuel tank.
Cheers