New build- cc rust removal and seasoning

113 gal propane motor fuel tank- I'm not moving very fast on it, slow project as time allows, but I've had the lid, firebox opening, stack cut out for a while, long enough for it to get some good rust on the inside- I've used an angle grinder wire brush to get as much of the rust out as possible, should I be concerned if I've missed a spot or two on the rust or propane residue? What else should I do to get as much of it out before I put a good layer of veg oil and light a good fire in it to season it?

I'll season again after I get the firebox on and rf plate and all in, I just wanted to once while I could still reach and brush out the entire cook chamber. 

Am I going about it right?

Thanks