I've never worried much about a bit of varnish on my engines. I'm also a firm believer in the concept of if it ain't broke, don't fix it. If I have an engine that's running good, the last thing I want to do is something to mess it up. I've seen way too many instances where someone decided to take apart a perfectly good engine and the thing never ran right again. Once it's running well, I wouldn't do anything that would mess with the fit, including removing varnish from the piston.

But anyway, having a nice clean engine is important to some people. I haven't heard of anyone using Dawn, but the crockpot method seems fairly common. It's important to get the right type of antifreeze. It's also critical that you NOT do this inside or in a location where children or animals could get to it. The antifreeze, including the fumes, is toxic. Also, the crockpot will absorb some of the antifreeze, so it should never be used again for food.