No secret, Alumiseal.

At 200K miles you're not driving much farther without taking it all apart anyway. So a Band-Aid will do just fine.

I have to laugh, I'm also using a propylene glycol/ethylene glycol mixture in my '64Dog, it's about half green Prestone and about half RV water system antifreeze, $3.49 at my local Wally World for the 50/50 mix. It needs some real antifreeze to lubricate the water pump and do the anti-corrosive bit, but it is safe to 50 below. The color also looks bad, the green and pink don't mix into a pretty color!

R.