Advanced Auto sells the Purple Power De-Ruster in 1-quart liquid bottles or in a 1-gallon jug. It was recommened to me on the vintage-vans.forumotionDOTcom website.
The next step was to get a three pack of tooth brushes, some wash cloths and painter's foam brushes for a $1 at the dollar store.
AutoZone sells the Rust-oleum Truck Bed Coating in a 1-quart size. The guy at the counter mentioned that this brand does two things, it stops rust and has less of the "burrly" (textured) look and feels of the major truck coating brand that they sell.
The project had to be thought out first. This De-Ruster can strip paint too! So, with that said, any slits in the rain gutter at those rear clip/clamp joint will allow the solution to leak through and strip your paint. My van already has rusty paths from these joints andways. I had to use some basic plumber's putty to hold it over for the work and later to remove it for painting.
My van rain gutters did not break through the metal. So, the following steps are more of preventive or just got to it in time before things got seriously worse.
1. Wash the roof and rain gutters very well on a sunny warm day.
2. Inspect the rain gutter all around the roof for slits or seam leaks. If found, temporay patch them with plumber's putty. At the front of the van plumber's putty up the open end rain gutter on the A-pillar for both sides.
3. If your van is newer, either cover with painter's tape or remove the third cyclopse brake light assembly. I taped mine really good. Later I'll have to treat the rain gutter lip just under this brake light.
4. While on a latter at the rear of the van passenger side of the rain gutter, carefully pour the contents of the Purple Power De-Ruster into the rain gutter. If you have the 1-gallon jug, get a smaller container to pour with. Pour slowly as the rain gutter will fill up and star rolling to the front of the van. (NOTE: check with a level if the van is slighly lower to the front.) It will take about 1/4 cup or less to fill the side. BUT, do NOT overfill.
5. With careful movements, use the tooth brush and scrub the pitted surface rust inside the rain gutter to the roof of the van from front to back. Repeat a few times if the rust is a few layers on the surface.
6. Let the Purple Power De-Ruster sit in the rain gutter for about 30-45mins. The rust will convert to nearly pure white or to the primer color that is under the paint.
7. As the Purple Power De-Ruster evaporates a little and the rust converts away, you can start to mop up the Purple Power De-Ruster very carefully with the small wash cloths. Just have a good throw away plastic bin handy to toss these wash cloths out. FYI - Purple Power De-Ruster will eat holes in concrete.
Once the Purple Power De-Ruster is mopped up and the rust is converted, you are now ready to prep the rain gutter area for painting.
8. Shake the Rust-oleum Truck Bed Coating can a little while. More is better.
9. Using painter's tape and a ruler, mark-off from the rain gutter lip towards the roof 1-inch. Start and apply the painter's tape at arm's lengths pieces and 1-inch from the rain gutter lip to the roof surface.
10. Tape off the under area of the rain gutter. That is, the verticle vehicle side body panels and leave the rain gutter under side ready for painting.
11. Shake the Rust-oleum Truck Bed Coating can a little while. More is better.
12. Open the Rust-oleum Truck Bed Coating can, climb the ladder, and use a thin and narrow foam brush to coat the inside of the rain gutter. Paint the rain gutter trough first about an arms length and then up the roof to complete that one inch mentioned earlier.
13. Paint the rain gutter lip.
14. Paint the rain gutter under side.
15. Allow two hours for proper drying.
16. Carefully peel the painter's tape back