As mentioned in Heater Core Removal and Repair, I have decided to try and match the factory replacement finish of the heater core, which is painted black. The issue I have with the current state of the core is that there is a large amount of corrosion and oxidation. Although no one will see it once it is installed, I would like to remove as much corrosion and oxidation as is safe and also prevent more from forming.

I searched the web for a good method of cleaning copper heater cores but found no clear process. I was hoping to find a spray that would do the job, much like a coil cleaner which I use on aluminum radiators.  Instead, I found a method of cleaning copper wires that seems to be widely used. The process called for dipping the copper wire in a vinegar and salt solution until the copper was etched, and then neutralizing it in a bath of sodium bicarbonate (baking soda). I figured I would give this process a shot on the heater core. I cleaned up as much of the core as I could using scotch brite, although this only meant the end caps and tubing. The fins themselves cannot be cleaned using any abrasive as they are extremely delicate.

I setup a vinegar and salt bath in a plastic tub. I added enough vinegar to submerge the heater core, but not enough for it to pour into the tubes. I added salt until it became difficult to dissolve, which seemed to be about 3-4 tablespoons. In the sink next to the salt bath I made a baking soda bath. Unsure of how much baking soda was needed, I added as much as I could. In the end, the idea is that the acid in the vinegar will clean off the metal and oxidation, and then the baking soda bath, which is a base, would neutralize the acid and prevent it from continuing to etch the metal.

I placed the core in the vinegar and within a couple seconds it started to react. Small bubbles started to form and the vinegar began changing color. Most articles I have read recommended keeping the copper in the acid for 10 minutes. I kept watching what was going on as I waited. After about 3 minutes I started to notice something odd that I did not expect. The heater core has a frame that surrounds the copper fins with three bars that go across the width of the core. These are the pieces that I noticed to be rusting which suggested that they are made of steel or material that is not copper. A black residue began forming around the three pieces spanning the fin section. Not only that, the black residue started to form on the fins that surrounded them.

I should have known something like this might happen. All the examples I have read talked about cleaning copper, not a mix of multiple metals. One of the articles showed how after etching pennies in vinegar, any steel that is placed in the bath will become copper plated. I was never great at chemistry but clearly there was a chemical exchange occurring between the copper and the steel (or maybe any zinc plating that might have been present). My only guess is that the iron or other ions that were going into the solution were getting deposited on the copper.

When I removed the core after about 9 minutes and placed it into the baking soda bath, there was a nice fizzing effect when the acid came in contact with the base. I let it sit in the baking soda bath for a couple minutes and then I removed it to dry. After it dried I inspected the core to see how this worked, or didn’t work.

Around the three frame pieces there was orange rust like residue present. This same residue was present on all the fins that were on the bottom of the vinegar bath. Seems like the iron ions were heavier and settled to the bottom of the bath and deposited itself on the fins. This orange residue (likely iron oxide) was well attached to the fins and would not wipe off. The other copper pieces of the heater core cleaned up very well. There was black residue present on the repair material which is likely lead based solder. I would say that this process was a 50-50 success. The copper did clean up well but the iron oxide on the copper was not what I was looking for. Luckily the iron oxide was only in some areas and not everywhere.

I used some scotch brite to clean up the residue that I was easily accessible. I also used a delicate wire brush to try and clean the iron oxide off the fins, which didn’t work very well. I cleaned off all the surfaces with acetone and moved onto the painting process.

The opinions on painting the heater core vary. Some people think it’s a good idea, while others cite potential issues with the paint peeling or it restricting heat transfer. There has been testing performed on the effects of paint on heat transfer. The testing will depend on the type of heat transfer that is required. For a heater core the primary goal will be heat transfer through convection, as that directly warms the air which is the intent of the heater core. Radiation and conduction will also increase the air temp but they are smaller contributors. The testing that I have read has indicated improved overall heat transfer with the use of paint. Link to Heat Transfer Testing  Although that does not tell the full story, it’s enough to convince me to give it a shot.

I decided to use Eastwood Gloss Radiator Paint. This paint is rated to 300F where most traditional paints are rated to 180F, which is on the limit for an automotive radiator. The paint is also claimed to spray on much thinner than traditional paints, helping prevent excess paint buildup. I ended up doing two coats on each side of the heater core. The paint did spray very thin making it difficult to over apply. On the fin sections I had to paint at an angle to ensure the paint adhered to the surfaces and no just blow right through. The results were very good. The paint is thin but covers well. It’s not as glossy as I expected but that’s not a bad thing.

The last thing I will want to do is a pressure check the core prior to installation. Although the radiator shop tested it, it would be best to re-test it myself to make sure the cleaning did not open up any holes. I am thinking it will involve a water bath and some pressurized air, but that will happen closer to installation.