Car won't start. Mechanic says spark plug keeps fouling out but can't figure why

I don't know. He didn't say. Just put another spark plug in it, said it was almost 20 years old and has 283,000 miles on it. He also said he didn't have a lot of time. I don't think he did those tests. I think he gave up! He mentioned water in the gas tank and to put in an additive which I did.

Thank you for your help. I'm sorry I don't know anything about cars (my car). I'm taking it to another mechanic tomorrow. He will check if it is oil or gas fouling out the plug. He gave me several different scenarios of what it "might" be...not too bad to rings or head. Not knowing much leaves me at the descretion of others who hopefully are honest! Thanks so much again for your assistance.