Best way to clean underside?

[autoxcuda]

sand/soda/glass blast will ultimately give you a clean service paint will grip to.

 

[ckessel]

If you go blast method, I learned a trick many years ago. Wear a 55 gallon trash bag. Keeps the grit out of your hair/eyes/nose etc plus covers up most of your torso. Put a couple of tears in for your arms to stick through. You can see through them fine.