Paint needs removing with caustic soda NaOH solution (CARE!) or proprietary paint stripper.
Nickel plating can be removed with emery paper if loose, or (I am told) by boiling in sodium sulphate solution (I have not tried this).
Rust can be removed by first scraping, 400 grit emery paper or rubbing with wire wool, then treating with acid e.g. phosphoric, hydrochloric or a proprietary solution such as Jenolite or Hammerite. See Philip Bond’s excellent article on this website Philip Bond's tips for restoring Meccano.
An alternative method which is less physically demanding is to do this electrolytically:
This involves making the Meccano part the cathode (negative) and using scrap steel as a sacrificial anode.
The anode (positive) can be any old bit of iron or steel (but not stainless steel as dangerous chromium ions are released into the electrolyte if this is used). The Meccano part is only cleaned in line-of-sight to the anode, so use several linked anodes to surround it.
The electrolyte is a mild alkali such as washing soda (sodium carbonate) or dilute sodium hydroxide.
Only a low DC voltage is required. A 6v battery charger worked well. Check for short circuits with a meter or continuity tester before switching on the power.
Hydrogen is released which is potentially explosive so make sure ventilation is good!
The time required depends on many factors including the current and depth of rust – reckon on 24hr plus – just wipe with a rag every so often to check!
The Meccano comes out looking black and needs cleaning with fine steel wool and drying rapidly with warm air. It is very prone to rusting at this point so should be degreased and primed or plated immediately. Alternatively it can be stored in oil provided this is removed before any final coating later.
There are plenty of instructions on the internet, including YouTube, mostly from USA.
This one is UK and a good summary;