I had a whole write up written, then lost my online connection and its gone!
From a retired welder that repaired rusty frames in northern IL, you can relax a bit, what you have isn't going to fall apart tomorrow, or anytime real soon, unless you load it to the max, every time you use it.
Most modern frames are sealed pretty well at the bottom. The problem is, there are holes in the frame that water (and salt water) can get in, but then there is no way for it to get out, until the water level reaches the lowest hole in the frame side, usually about 1/2 way up. The frames tend to rust out from the inside outwards. You don't want to cover the outside of the frame until you can seal the inside of the frame.
The good news is, now your frame has a drain hole at the bottom. But to take full advantage of that drain hole, you need to get proactive pretty quickly.
Take a chisel along the rust through line and knock off the loose edges, open the hole until you get to good metal. Once you have a big enough hole, you can start digging out the debris that is sitting on the bottom of the frame rail. You want to dig as much of that carp out, as far and as wide as you can. Use a screwdriver, or similar item to scrape along the edges and rake that junk out. Its likely you are going to see pretty big chunks of what look like frame rail (because it was before the water and salt started breaking down the steel). That frame rail remains is thick, flaky, and breaks pretty easily, dig it all out! You are probably going to be amazed how much crap you will dig out of your frame rail.
Once you have it pretty well cleaned out, get out your garden hose. Find a hole about 2' to 3' in front of the new "drain hole" and insert the hose, pointing the water towards the drain hole and flush the frame until the water comes out clear. You may have to stop and dig more crap out of the drain hole before you get the frame clean. Repeat the flush process to a hole about 2'-3' behind the drain hole. You do not want to try to flush past the frame kickups.
Once the frame is clean inside, let it dry for a couple of days. then you can seal the inside with paint or what ever you choose, just be sure the frame is dry before you coat the inside.
The bad news is if only one side has the rust hole, the other frame rail still has all the crud inside of it.
Once its sealed inside, an annual wash and maybe a new sealing coat will extend the frame life a long time. You may not have to have it patched for years. Gene
