I would clean good for sure otherwise no chance.
Now can this get sidetracked to a political shitshow like most the other post? Lol
Sure it can! Just change the title to say White Flexseal and sit back/watch!
On the serious side though, this stuff is pretty amazing.
Did not use it on coolant yet, but recently sealed up a Wire EDM water tank that is made of steel ( whatever Brother was thinking with the HS3xxx series )
I've used plastic mosquito net to build up a few edges and patch seriously large ( 1-2" dia ) holes.
There was no way to absolutely clean up the rust, so did the best I can and then brushed the stuff on it.
The one I used came in a can and it says it's the "thicker" formula, but it's still has a pretty darn low viscosity and takes a bit to dry.
I ended up brushing the first layer onto the screen patches, and kept on going back every half hour to an hour to make sure there were no big drips.
Then the second day ( some 18 hours later ) it was cured to a consistency that was still fairly soft but no longer pliable.
I then added a second layer with brush-on and extras where needed and let it flow around a bit.
Then after another 24-32 hours of curing it became thick and fully adhered. I have tried to pry it up in some places to see how well it stuck. Well, It was stuck!
In any case, I would recommend that you go ahead and try, but please get back to us about how it worked with coolant!
In my case with plane water and rusty surface, it is flawless.