Hi there, all,
I second much of what Fergus has written.
When I bought my drilling machine, it was caked with thick grease reinforced with fine brass swarf. I scraped off as much as I could with a variety of instruments (wooden spatula and iced lolly sticks) and then soaked the castings in hot caustic soda solution. Followed up with several good warm water rinses and dried by brushing with methylated spirit.
That got me back to a paintable surface. This was back in the early 1970s and I don't remember what I used for a primer but my top-coat of choice in those days was Valspar 3-hour enamel. Or was it 4-hour? It doesn't matter because they subsequently changed the recipe and then I think it disappeared from the market.
I believe the original Valspar was alkyd resin based. I was told that the traction engine restoration folks would dilute Valspar 50/50 with genuine turpentine and then flow it on with a sign-writer's mop (that's actually a soft brush). It would self-level and then dry to dust-free in seconds! I guess you'd have needed several coats.
Another thought - there's a place in Portsmouth called 'the strip joint', they dip old doors or furniture in a stripping solution that removes the paint. (It also removes the filler and the glue!!!) The process is preferable to sanding off the old paint, especially if it's old enough to be lead-based. I guess there are similar emporia in most big UK towns & cities.
If you have them in
Sweden Finland, maybe one of them would take on your castings???
Edit: I'm sorry, I got your country wrong. I hope I haven't caused an International Incident!!