I had a lawn tractor engine freeze up, and I pulled the head to fix the problem. While I had it apart, I put several parts into my ultrasonic parts washer to try to remove the caked on dry grease. It cleaned the parts really well, but apparently removed the surface coating of the metal that prevents rusting. Totally exposed, the parts began surface rusting in less than a day -- not good.

What can I coat the metal parts with to prevent surface rust that can handle the heat of a small engine? I've got clear electrical varnish, WD-40, petroleum, silicon lube, gun oil, etc. What works best? Any "don't use" protectants that would adversely impact the engine?

Bonus question: The ultra-sonic parts washer is a new acquisition. Are there things I shouldn't clean with it? It seems to remove essentially everything on the surface of metal, both the nasty stuff, and any protective coatings, paints, etc.

  • Do not try cleaning irreplaceable jewelry with the ultrasonic cleaner. Something you thought was solid gold/platinum/silver/heirloom/memory may prove to not be so. Commented Jul 4, 2017 at 22:20

2 Answers 2


Are you talking about protecting the parts from surface rust while the engine is disassembled and being repaired? If so, I just use light machine oil (like 3-One) brushed on with a cheap chip brush. WD-40 will work also. Whatever film remains will be washed away from machined surfaces when you prep them for assembly by cleaning with denatured alcohol. Your fresh engine oil will wash away oil film too, remember that you will change the oil during the break-in process.

  • This is what I'm looking for. Just something to prevent rust until I get everything back together. Mostly internal parts. I'm confident that once running, the various petro fluids will keep things protected when it is assembled. But, I don't want the valves, pushrods, etc filling the internals with rust particles.
    – user49831
    Commented Jul 6, 2017 at 21:43

Any external exposed engine parts that are steel or have iron in them probably need to be painted with a high temperature engine type paint. (You can get it in a convenient spray can at the auto parts store).

Anything that is going to get hot from the engine operating you do not want to use an oil or other petroleum based compound for a coating. A clear varnish that is made specifically for metal parts would be suitable. WD-40 --- Never.

  • So... you're saying he should paint the machined surfaces, like heads and pistons?? Commented Jul 4, 2017 at 21:25
  • Obvious not to paint the inner working of the engine. That would not make any sense at all.
    – Michael Karas
    Commented Jul 4, 2017 at 21:28
  • So why would one be worried about superficial surface rust on the outside areas of a lawn tractor engine. I (perhaps mistakenly) assumed he was asking about machined engine surfaces, which are susceptible to quick surface rust. Commented Jul 4, 2017 at 21:31

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