Re: Thin hairline cracks in Plaster Paris - how to fix?
The problems that accrue to using patching plaster by far outweigh the very slight speed advantage/ 1) no chemical bond to the underlying lime plaster browncoat, or the lime & gauging whitecoat. It will hold only by a mechanical bond.
2) No tensile strength.
So I recommend either patching with lime & gauging, over mesh tape (for tensile strength), and prime the cracks with a bonding agent (Link or Plasterweld); Or use setting-type mud (Durabond 90) and again, spray, roll or sponge a bonding agent into the cracks to seal and prep the repair so you get a decent shot at a strong bond. Once you gat a coat of Durabond on the repair, you can finis skimming with regular green-lid drywall mud, or trowel on some Easysand 45 (like Durabond only a bit more sandable) to feather it out. In a wet area, (bathrooms) stick with Lime & Gauging or Durabond, they are more forgiving of moisture.
Remove not the ancient landmark, which your fathers have set.