Rob,
I can't speak to weak spots in the chassis (I look forward to reading those who know) but using Por-15 is a dandy idea. If you don't know (after it is sand blasted), give it some good blasts with compressed air or vacuum the heck out of it using the brush and corner tools from the vacuum. Then, and this is big, keep wiping down the metal with the Por-15 thinner until the rags are clean.
Por-15 leaves no film like lacquer thinner. I use the semi-gloss not chassis coat. And, after I pour it out of the holes I punched in the lid (do not remove the lid and use wood screws to re-seal the punched holes) I let the paint "gas off" for about an hour. This thickens the paint and gives me a better one coat finish.
Just an opinion. I sold the stuff for ten years........there is nothing better.
mike