Had the boat sitting on the trailer for the last 3 years. Used RV anti-freeze when I winterized. Fired up one eng recently and out came a bunch of brownish water that stinks and stains gelcoat. Thinking the pink juice didn't really help much with corrosion prevention. Started looking around for an easy way to clean out the block cooling passages.

I've used Ospho (phosphoric acid based) on some really corroded metal pieces with great results. It turns the iron oxide (rust) into iron phosphate - an inert, hard substance that turns the metal black and provides a bit of corrosion resistance. However, I also saw one report online where it was noted the rust turned black but swelled up and blocked the water passages.

Started look around some more and found a product made by Evapo-rust. Sounds like it may be a safer option. Anyone tried this stuff before on a raw water cooled eng? Any other ideas?

https://www.evapo-rust.com/thermocur...-rust-remover/

Evaporust can be made with powdered citric acid, baking soda, and water. + a bit of dish soap for a surfactant.

1L water
100g powdered citric acid
64g baking soda
+ small bit of dish soap.

Add citric acod to the water....mix.
add baking soda...mix and wait for the reaction to stop.
Add the small amount of dish soap.

You can get bulk powdered citric acid.
Pretty cheap to make vs Evaporust, amd it works better/longer.

I wpuld dunk engine blocks amd parts in it...not sure I would put it in the engine. Don't know how it reacts to aluminum, or water pump parts etc.

Big iron scale is an issue. Evaporust type chemicals habe a hard time on advanced iron oxide in layers etc. Takes time.

I was a bit worried abt the circulation water pump using phosphoric acid. The stuff in the link I posted is designed to be poured into the cooling sys and drive around for 3-4 days. Should be safe for alum/brass but I can't really follow the drive around instructions w/raw water cooling. Best I can do is drain block and then fill with the stuff in the link after premixing it with water. After a few days drain it out and flush.

At work we use cascade dishwasher machine soap to clean out cooling systems that have been contaminated. Dishwasher machine soap doesn't foam so you can run it for a while and get it warm.

The cascade sounds like it would probably work good on a closed cooling system. I suppose with raw water cooling, one could drain block, fill with cascade/water mixture thru thermostat housing. Start eng and let it warm-up some but not enough to open the thermostat. Maybe repeat that a couple of times, then let it fully warm-up, so thermostat opens and flush it all out.

However, I'm pretty sure the old RV anti-freeze yuck will all be blown out on the first trip to the lake. It's the rust I want to try cleaning out.

I'm trying to save the 35 yr old blocks from cancer (rust). After more thought, I'm now wondering if I can use something to remove most of the rust (Thermocure product in link?). Then after that's all out, drain water and fill block with a phosphoric acid product (Ospho), let it sit for a few mins and flush it out. The remaining surface rust would be chemically converted into iron phosphate and help protect the cooling passages from future rust.

Abt 10 years ago I made a tool to remove a stubborn cap on a pool pump. It was made from bare 2" square tubing. I left the tool laying out by the pool equip and it rusted severely. Deep intergranular rust. I knocked off the loose rust and coated it with Ospho. The remaining surface rust turned into iron phosphate. That was abt 2 years ago and it's been laying outside in the elements ever since. It hasn't started rusting since then. That's what gave me the idea abt using it in an eng block.

First time I used the Ospho stuff was in the mid 1990's. I had built a carport out of 4" blem well pipe. It started surface rusting soon after being built. Applied the ospho with a paint brush to the pipe. The pipe turned black and hasn't rusted since. So I know that stuff really works long term.

Im having a cooling issue right now too and Im going to use the Enviro rust stuff too. I think my exhaust manifold is rusty inside and the chunks are blocking the turbo shields and tail pipe water passages. It says that it is safe for rubber. Im going to drain the manifolds and then lift up the hose at the front of my exhaust logs and at the back and pour it in and let it sit for several days. I bought the 5 gal bucket so i will see how much each side holds. Ill come back on here and give a update. I was running my twin turbo scarab the other day and one manifold was 116 deg vs 150 and one turbo shield was 128 and the other got up to 450 so something is definaterly blocking one side of the motor. And this is on a 45yo 454

Certainly does sound like you have some blockage. I'll definitely be interested in your results with the exh manifold.

Think I'm going to order some of the Thermocure stuff today. Still on the fence with the Ospho treatment.

Thinking out loud, if I can get rid of most of the rust with the Thermocure, run the boat a few times so a thin new layer of surface rust develops, then hit it with the Ospho. In theory that will provide a somewhat uniform protective layer between the water and the iron and help prevent future rot. Heat transfer was a concern initially, but I figure is likely no worse that the layer of rust I already have. I did see a warning abt using Ospho on hardened steel. As with any acid, it can caused hydrogen embrittlement. Probably should fire off an email to Ospho and see if they have any experience with this.