There are several ways and opinions about how to remove cosmoline, and many curses heaped on the head of this product over the years.
Cosmoline is not evil, if it was not for its preservative effects, we would not be able to enjoy shooting old military surplus guns as they would not have survived over the years.
You Need to Understand Cosmoline
Before learning how to remove Cosmoline, you first need to understand that it is chemically similar to Vaseline, and is applied by dipping guns into a vat of molten Cosmoline. This means that the preservative is not just gunked up on the gun, but is embedded in every nook and cranny in the gun. If you are going to remove cosmoline from a gun, you will have to disassemble and detail clean it.
Some like to use chemicals to clean out the petroleum based Cosmoline. I have read accounts of people using gasoline a 55 gallon drums. I think that this is overly dangerous and under-effective. Mineral oil and brake cleaner work just as well.
Hot Water Method
I personally use hot water for the metal, and sun and gently heat for the wood. Some do not like the idea of using water, but in my experience using heat alone runs the risk of cooking out the oil and leaving the dark tar-like crud. Water seems to both heat the oil and help float it away.
The way I do it is to strip off all the wood, and disassemble the gun to is smallest user level parts. I don’t do an armorer level disassemble, but just a detailed field strip. I then put all the small parts in a stainless pot that the wife won’t kill me for ruining (I actually have my own kitchen set by now), and boil them clean.
The Cosmoline will float to the top as it melts. When I take the hot metal out of the water, and quickly clean it with bore solvent, it dries rather quickly and I oil it well so it does not rust.
The longer parts like the barrel, takes more work.
I boil them in a large stock pot, and repeatedly pour hot water down the barrel to loosen up the Cosmoline. A rod will need to be pushed down the barrel as it will be plugged with the Cosmoline.
Attention to Detail Helps
Take special care on the action, as with guns such as the Mosin Nagant, In the video I show a Mosin, but this is not a how to remove cosmoline from Mosin Nagant article.
Cosmoline is notorious for being hard to remove. It may cycle fine, but after a shot or two, the Cosmoline will become tacky and the bolt will be hard to cycle. Additionally, If you fire the gun a lot with the Cosmoline on the action, it will bake on and make a small problem a huge nightmare.
I take care with the stock, and do not introduce boiling water as that will damage the old wood. What I do is to gently heat it up in the sun on a hot day (or VERY gently heat with a hairdryer) and wipe the Cosmoline off with a towel. With repeated heatings and wipe downs you can remove the Cosmoline without damaging the wood. If you go to fast or too aggressive you can strip out the moisture and mess up the stock. I also use murphy’s oil soap to help remove the oily Cosmoline from the wood.
I know this is not an easy process, its messy, and will most likely cause a little bit of marital stress, but look at it as a rite of passage, and a way to help preserve history. Heck, if you get a C&R license, you may even consider buying a curio gun and leave it in its Cosmoline wrapping to allow your kids and grand kids this pleasure.