Penetrant test

Steve welder said
I use WD 40 all the time but never for rust penetration. I buy a gallon and use a sprayer bottle and I find this economical and a better application
Kroil cans, not the spray, and Never Seize cans were literally everywhere to be found in the powerhouse during the 20 plus years I worked there. It was a staple, that they never ran out of, cases upon cases of this stuff, that and Permatex #1 &2.
Your talking about extreme high temperatures, steam, salt water used for cooling, all kinds of equipment
However many parts were so frozen, only heating red with a torch would work
All fasteners had anti seize applied, that was standard procedure
When I went into ship repair, once I was standing on some below deck stiffener, I was about waist high to the decking next to a main engine, it was a tight area.
I had to burn something, I forget. What I didn't know was someone dropped a WD 40 can below me.
The molten metal from what I was cutting hit that WD can and pierced it. Flames shot out about 8 feet inches from my body and face. I was like, what the F is this. The guy above me ran for a fire extinguisher, I couldn't really move, the flames died out has the pressure in the can got weaker
Another learning experience

This story reminds me of a funny(now) story. Years ago, my wife worked in a bank check processing center, and they were getting rid of a big old 110v 10key mechanical calculator/printer which hadn't been used in some time. She brought it home for me to use for taxes, but it was kind of gummed up with oil & dust. So I sprayed the number stamp keys with WD40 and then hit the total key, again and again, and it was beginning to free the stamping keys up. Spray, hit the total key, repeat. It was working but slowly, so I got impatient and held the total key down and sprayed at the same time. Worked great, until there was a bright flash and flame and I smelled burning hair. Lost most of my eyebrows, mustache, & hair on my fingers. Like I said, it's funny now, not so much at the time.

As for the penetrating oil, I've used most of those and they are pretty good, but my fave is Mopar heater control solvent. It was originally for exhaust manifold heat risers, and exhaust manifold to head pipe bolts.