Reddit mentions of Skyco Ospho Surface Prep

For rust spots that are rotted through completely, you'll definitely need to cut them out and make patches. This is hard, but with lots of patience and the right cutting tools, a little bit of welding skill, you'll be fine.

Remember that for welding on body panels, the sheet metal is very thin, so weld carefully to avoid putting to much heat in. Too much heat in a body panel will warp the surrounding metal and cause an ugly look when you do body work, making it hard to smooth out the area even with bondo. Try making many small spot welds. If you have an air compressor, a good way to cool down the metal while welding is to attach your 'blowing' attachment and just blow cool air over the region after each weld. Patience is absolutely key. Have a steady hand. Do not just burn through on the body.

When welding on frame pieces or other things that won't be seen, this is not as important, though you should still be careful. A fire is not great.

I would suggest you buy a large sheet of fairly thin steel, a gauge which matches body panels closely. Too thin and you'll burn through; too thick and it will be hard to weld.

For surface rust and that rust which does not rot through panels, rails and other metal completely, use Ospho. Ospho is a compound of phosphoric acid and a few other additives that, when added to rusted metal, causes a chemical reaction which hardens into a kind of undercoat. When using Ospho, make sure that you clean up the area first with a wire brush and, for best results, some sandpaper as well. You'll want to knock off as much scale as you can before applying the chemical so that it works properly. Applying Ospho to scaly surfaces will not allow the chemical to be effective.

Here is a link to the big bottle. I'd suggest it since you can use it again in the future. It is to be applied LIGHTLY. Noting this, it is suggested that you buy a heavy set of paint brushes. Have a dedicated rag. Finally, get a spray bottle that can handle corrosive chemicals. You can find these at your local hardware store.

OSPHO IS DAMAGING TO PAINT. OSPHO IS DAMAGING TO PAINT. OSPHO IS DAMAGING TO PAINT. DO NOT APPLY TO PAINT!

When using Ospho, you must recognize that you are using a corrosive acid. As such, it is recommended that you wear nitrile gloves. Always wash your hands with a good basic soap, like joy or dawn, after using the chemical. This will neutralize the acid. Do not rub your eyes. If you get a little on your skin, you will be ok. Just wash, wash!

If you are not comfortable with bondo, find a good body man. A good body man is hard to find. Choose carefully your body man.

These are some hot tips. If you need help, feel free to message me.