After lots of trial and error I finally have a satisfactory paint finish.
What I have learned using 2K automative paint (noted here for my own future reference):
1. Use a touch-up spray gun, not an airbrush.The airbrush results in too much overspray. Set the spray pattern to be as concentrated as possible.
2. Take all necessary safety precautions. 2K paint is poisonous!
3. Clean all surfaces thoroughly with a pre-cleaner that does not leave any residue, such as benzine (not acetone).
4. Mask with good quality masking tape. Remove all masking tape residue where tape is removed. Take particular care when masking right up to the edge of a surface, such as for the valve bonnets. The edges to be painted must be perfectly clean for maximum paint adhesion.
5. Wear latex gloves when handling cleaned parts and also while masking.
6. Dilute etch primer 50/50 with thinners for spraying.
7. Allow ample time for etch primer to cure.
8. Dilute 2K colour coat 50 (paint and hardener) / 50 thinners.
9. Spray at least three coats, allowing the required cure time between coats as per paint instructions.
10. When spraying, hold the parts to be sprayed up in the air, away from any dusty surface.
11. Allow at least 48 hours for paint to cure.
12. Rub all painted surfaces with ultra fine steel wool to remove any dust particles and/or orange peel.
13. Be very careful with sanding sticks. It removes paint on small surfaces very quickly.
14. Spray two more colour coats and cure for 48 hours.
15. Lightly rub all surfaces again with steel wool to obtain a smooth surface.
16. Polish all paint surfaces by hand with a lint free cloth, using an automative polishing or rubbing compound.
17. Use gloss clear coat to protect metal surfaces not receiving a colour coat, such as the fly wheel rim. Rub the gloss coat with steel wool for a semi-gloss finish, or polish with rubbing compound for a high gloss finish.
And everything back together again:
The very last bit of work on the engine was to secure the small brass oiler caps on top of the glass tubes, otherwise they will disappear very quickly.
The best solution I could find was to punch out small pieces of double sided tape, normally used to mount mirrors against a wall. The tape is glued to the cap, and fits tightly inside the glass oiler tube.
And that concludes the build!