Looks like somebody left it with water in there I would replace that thing. You'll never get all that crap out of there no matter what you do and it's just a matter of time before a little chunk breaks off and gets lodged in the water pump stoping it and then burns the whole engine up trust me on this trying to reuse radiators that have that sand crap coming out of them is definitely not even worth the trouble.
Last time I drove around all over town looking for a radiator shop that could clean those out everyone told me the same thing they don't do that anymore and it's cheaper just to replace the radiator.
You can try filling it up with gas letting it soak for a couple days then drain it out then fill it up with CLR let that sit for a couple days then flush the crap out of it with a garden hose that would be my last resort if you can't find a new one...
Faster then a speeding E Revo
Last edited by MrKitty; 12-05-2019 at 11:59 AM.