Thanks for the kudos MandalorFett, and I did read some where the reference to that detail being called "piano keys".
I haven't fully sealed all of the cardboard just yet. The tube sections have a coating of Bondo spot and glazing putty on them, which was used to smooth out the surface to get rid of the "faceted" look. I plan to pick up some polyacrylic sealer, which will seal the cardboard and effectively make it waterproof.
As for the main bulk of the jet pack, it should be just fine one a fine coat of primer is applied as it's made from hardboard/wood 1/8" thick. I've painted this material before with no ill effects what so ever.