To paint or not to paint the underside

I wasnt sure what to do with 69 but needed to protect the new metal without sinking a fortune into it. I ended up rolling/brushing on Armor Coat semigloss.(owned by Rustoleum I believe).
But I looked into a few different options. I bought some paintable undercoat kit, I'll get pics in a bit.
Sandblasting could snowball quick and it would be EVERYWHERE. I bet shop wouldnt guarantee not hurting paint job. Would you consider just coating/painting over top of what's there?