Can I use a spray sealant to seal a leak in my radiator?
From reading on the Internet, FlexSeal looks like a scam ($59.70 for shipping, signs you up for $29/month "membership" without asking). It looks like it's similar to automotive rubber undercoating, which you can buy locally for ~$5 per can.

Don't know if it would work for your purpose. But I can recommend J-B Weld. Sand and clean with acetone, then apply a thin (1 mm) layer of J-B Weld. See also the answers to this question.