Cleaning a spray gun - like new

Any secrets on cleaning spray guns used for spraying pre-cat lacquer. I'm talking here about once a year spring-cleaning type cleaning - getting it back to like it had just come out of the factory - not the sort of superficial clean routinely done at the end of a day's spraying.



I thought about soaking them in thinner for a day or so but have been told its bad for the seals - no idea whether thats true or not.



Cheers

Arron