The white round dot is some silicone sealant. I sprayed waxoil inside the frame and filled all the holes with sealant when I got the new chassis in 2015. My 2013 chassis had more underfloor alloy panels than this one from what I remember, certainly down the center. A visual inspection may not be so easy.
Many years ago I had a car Ziebart rustproofed. Some time later I could hear water sloshing around. I removed the plug in the sill & clear water gushed out. I removed the rest of the down facing plugs. Water lying inside the tubes is not good. Can you be sure you have coated 100% of the inner surface?
