Sand could be too large. Use #4 or fine sand. (Use respirator!)
Siphon blaster $30! Got mine for $10 at Wallymart.
I have used the same set you are describing and it is not going to be a fast process. I found the siphon blaster good for the nooks and crannies but used other methods (knotted wire wheel on grinder, DA sander with 80 grit paper)for removal of larger areas)
You might call around to local equipment rental outfits as some may have a pressure pot and all should have 175 CFM towable compressors. Compressors rent for $75 a day here and blasting pot rigs about $100. If you do go this route be careful as the sheetmetal WILL warp if you are NOT CAREFUL. That will leave you in a worse spot than where you are now.
