Rust in thermostat housing

[454cid]

I'd try to chip away at that with a hammer and screwdriver. I bet you knock some big chunks off.

Just not your favorite screwdriver!

 

[1989GMCSIERRA]

Most likely it’s been in water that contains salt or alkaline and/or the guy never flushed the boat or drained the water in the block.

you can probably soak it in diluted muriatic acid to get it cleaned. If there are big pits in the casting it’s trash. Most likely it’s on its last legs.