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

You can always put POR15 into a cheap sprayer and use it then throw it out. Don't worry about the rust on the frame being to the point where it weakened it. The truck is not that old. Trust me up here in the Midwest you see trucks who have bodies that are trashed from rust but the frames are still strong as can be. The rust-o-leam paint that they have out now in a rattle can is not bad at all if you use the primer for rust metal and if I was you I would use a metal etch to get that rust off for the most part and prep the metal better to get that primer and paint to adhere as best as you can.

Like Dice said also keep a can in the truck and when you notice a bare or scratched area or whatever paint it. Keep up with it and you wont have to worry about it much. Also keep the truck clean as you can after it sees salt in the winter. I know of some tricks for up here in the winter that people say im CRAZY for doing but it works and it works very well. Seen 20+ year old trucks and cars without rust that are driven in the winter up here.

 

I am up here in Canada and we deal with rust a lot. Salt on the roads and warm moist temperatures create incredible corrosion issues. Must trucks over 5 years old feature rusty frames. It's not a big deal. Simply get it sandblasted and then repainted. I have never seen serious structural damage on a frame. But I have seen rear ends rusted through!

 

POR-15 is the best stuff on the market. I did an entire frame on a 1970 chevy pickup with less than half a gallon. A little bit goes a long way and it is very durable and strong. If it is in sunlight a lot, it will get a bit hazy/milky colored but it's not a big deal. The last time I bought some about 2 years ago it was $135 for a gallon at the local paint shop. I didn't spray it because it will ruin your gun. If you brush it, it will leave brush strokes as you are applying it but as it drys, the brush strokes go away.