My 54HC deck has a solid metal stiffener bar nearest the ground. There's a gap behind it where debris & moisture can collect. This got me thinking. I know....I'll fill it with caulking!! LOL, no.
Filling it, however, if done correctly and cleanly with the right stuff, may not be a bad idea. So long as what's put in that gap is durable, paintable, and lasts long term.
3M 5200 Marine Sealant & Adhesive would be a worthy product. There is also 4200 & 4000. A little masking of things and some Isoprophyl are a must for neat and tidy
Just tossing my thoughts out.