brickboard,

my 89 240 dl is starting to rust down by the rocker panels under the plastic molding that runs between the the wheel wells. mostly the rust is at the ends where the wheel wells are.

i have used a 3m undercoating spray in the past that is really nice for under the car. it is kind of a tar and will most likely disolve with gasoline.

any advice on what paint i should use? i will probably do the whole strip and possibly the under lips of my wheel wells.

i have considered 3 products: por 15 with a brush, 3m undercoat spray, and good old spray rustoleum. any advice? thanks.

If you're looking for a quick and 'frugal'fix, use the rustoleum, but be advised, it's just a coverup.Cheap, but you'll eventually be doing it again(but it doesn't take long)I have used the spray undercoating and after it "hardens" have been able to paint it with Duplicolor for satisfactory results, but it,while a lot better,is still just a cover up. A very good one though and will last a long time.If you are a "do it once and do it right" type of person(anal retentive)Then the Por15 is the way to go. A lot more labor intensive,(in comparison)but it does treat the problem,not merely cover it up.
I've done them all,but if you're looking for opinions, do the undercoating/paint route.

I've had good results on rust spots with POR15. You need to very carefully prep the surface. First a complete degreasing as any oil will interfere. Then a Zinc prep solution (from the POR15 folks). Then the POR15. The POR15 dries very hard and very black. You will probably want to coat with your body color. Use throw-away brushes with the POR15. Also, if the rust is coming from inside the panel, you will need to deal with that or no amount of painting will last.