Re: requirement to paint white, it seems to be a thing. Example: How to install weight on kart (reply #3). Tape would be a no-go.
Re: taking the weights off. At my local track, the lead lap time in his class is 37 seconds. My son is around 50 seconds. You’re obviously right once he’s competitive, but right now, wife acceptance factor is most important… and 40-45 pounds of weights won’t help that cause. 
Thanks for the pointer to the instructions, I didn’t realize lead isn’t like regular metal, and needs multiple coats of special primers. I’d have to paint 80 pounds, looks like an all-day ordeal. Will need to look into the costs of the various sprays to see if this still even makes sense.