Exactly how big of a deal is rust on my frame...

+1 on por "pour over rust" 15. I fabed steel motor mounts for my boat, covered in por 15. 6 years in a salty bilge, no rust.

get the small cans. It does not keep well after opening. If you get it on you, it will be on you!

 

I owed a 1961 steel hauled houseboat made out of 1/8" steel. It lasted 40 years and it only had bad metal in the head where everyone was drunk and missed the john. That's kind of similar to road salt. The rest of the boat metal was fine. Rust is like cancer. It's slow spreading for many years until one day it's out of control. Yours is probably surface rust which will leave pits. It should still have it's structural integrity.

 

I used POR 15 in a body shop a few times, dont sand the rust all the way off it says it works better with some rust (Big rust scale- remove). Nothing to be scared about -but more precaution is needed if spraying.

 

I used to have a paper sheet with all the instructions for POR 15 - i never used any primer. I sprayed it on but from a new can, i cant remember thinning it (I may have thinned it with a very small amount of lacquer thinner) --they do make a thinner for it but i know we didn't use it. Clean up gun immediately with lacquer thinner --once it dries it isn't coming off --it dries to almost a ceramic coating. It dries best in a humid environment and will feel tacky the next day. If you are going to top coat it with something else (color) you have to spray a primer on top when the POR is tacky. Here is the problem with POR 15 --once you open the can it starts turning bad or curing. use an edge of a cardboard box to clean the rim of the can and seal it as soon as you pour from the can -turn upside down and refrigerate the can to store it, doesn't last forever -if you leave the rim filled with paint -you wont get it open again -it dries very hard.
As far as sprayers go --just get a harbor freight gun --for cheap --brushing it on is fine if you are not making a show truck - and like i said if you are going to top coat it -wait till it sets up well and spray your primer on carefully.
The black color dries to a opaque kind of black --not a deep jet black.
Here is my opinion on the frame rust and maybe in "Big Truck " land its different-- but stop it the best you can --if it passes inspection your good - but who knows what DOT cops have to say about it ?

 

You can go down to NAPA and get dust masks and a respirator. You don't need a grinder, just a wire wheel and I bet you'll see clean metal in a second. If you don't then you have issues no paint is going to fix.

 

Hang-Man has got it! I never sprayed it, just brushes. Got the cheapest ones I could find so I didn't have to worry about cleaning them!