There are so many varied industry guys on here with tips or tricks on different products, I thought I'd add one product that I have found outstanding over the years in my compressor reconditioning business - I hope it isn't against forum rules.
It is Orapi - Reduce 4.
It is without doubt the best spray on de-grippant/de watering fluid (loosens rusted bolts etc) I have ever used and we've tried most products. It was so much better than anything else on the market, we used to ship it by the box full from an Engine reconditioner in Rockhampton - the only place we could find it. Forget CRC, lanolin based sprays and wd40 etc.
It can now be purchased from any Blackwoods.
Leave an engine - no plugs/injectors in, outside for 4 months in the weather/rain - rusted solid, tip out the water and spray Orapi liberally down the bores and around big ends - tommorrow it turns over with a spanner.
I don't work for or have affiliation with this product or Blackwoods.
Really handy in the garage.
As Molly says - do yourself a favour