I'm working on assembling a groundplane antenna for my truck

Hi Jeremy, I guess you are assembling a mobile antenna for your truck. The
groundplane for it can be the metal body of the truck. No need to provide
"ground radials", although a groundplane at VHF, mounted on a truck has been
spotted by some folks.
Just use a simple 1/4 wavelength whip at the transmitting frequency, connect
that to the center conductor of your coax, and securely bond the coax shield to
the metal body of the truck. I would not paint. The mounting details are
usually the most difficult.
73 Gary N4AST