First of all, be very careful when sandblasting a flat surface like a door, even a fender but to a lesser extent, as you do not want to deal with a warped panel. I have never used a syphon blaster, so I cannot tell you how good it will work. Working properly, it should take the rust off though. At work, using a small pot blaster we have two 80 gallon 5 horse each compressors. Just enough air. At home for my own projects, I have the exact same small pot blaster, a 20 gallon-5 horse compressor. Can only blast constantly for a minute or so.
Next, alternatives to sandblasting. Is it mainly light surface rust? If so, find someone with an electric 8 inch orbital sander (or buy one). (I say electric because I doubt your compressor could run a pneumatic sander. I also say 8 inch because it just makes quicker work of the rust/paint). Electric works fine, at the body shop we use it over the pneumatic 8 inch ones. To strip down to bare metal, use 60 grit discs. It will strip your whole truck down without to much effort.
Hope this helps a little bit.
Good Luck!
Colin
