February 2022 Update

 Work has been progress on my OcCre San Francisco Streetcar and the blog on that will be updated accordingly.  I did find that I was missing one of the filigree corner pieces,  and decided that I would cast my own, and used this as an opportunity to have a little trial:

These are my results from my trial.  First the material used for moulds and castings, the numbers refer to each item in the photo, and my conclusion below the photo:

1. This is the Blue Stuff mould.
2. This is a 2 part silicon mould.
3. One of the original white-metal items used as a pattern.
4. A 2 part resin cast made using the silicon mould.
5. Two 2 part resin casts cleaned up.
6. Green Stuff cast using Blue Stuff mould.
7. Milliput cast using Blue Stuff mould.
8. 2 part resin cast using Blue Stuff mould.

The silicon mould [2] could be removed from the pattern after about 1 hour, however the instructions suggest waiting at least 24 hour before use to fully cure. As suspected it had a number of small air bubbles, however due to the fineness of the item being cast cleaning the excess from the cast was impractical though not clearly seen in the block [4], though some casts [5] from the silicon mould were acceptable. Both Green Stuff and Milliput cast were attempted from the silicon mould, but due to the flexibility of the mould I could not get a casting from either.
The Blue Stuff mould [1] was made, following the instructions, heating for 1 minute and ready for immediate moulding, the mould can be used as soon at it reaches room temperature. My first attempt was very poor, however due to it re-usability, a second attempt took place taking note of the reasons for the first failure, which was not using small enough pieces initially to form around the mould, also just before the mould was ready, I re-immersed it in hot water to complete the moulding.
Using the Blue Stuff mould I cast first with Green Stuff [6] and after rolling out thin it took to the mould, was de-moulded after about 3 hours, and took 24 hours to harden, though it still retains some minor flexibility.
Again, using the Blue Stuff mould I cast with the Milliput [7], applying it similarly as I did with Green Stuff, this was far more sticky to handle. De-moulded again after about 3 hours, still not fully hardened, with hardening taking well over 12 hours.
My last cast with the Blue Stuff mould was the 2-part resin [8] hard enough to de-mould in less than 1 hour, and hardened by 3 hours (this will depend on the resin you use).
Conclusion:

• Silicon is normally a good mould agent with 2 part resin, though it takes time to cure, and develop air bubbles and though the mould can be reused a number of times, you cannot make a new mould with it, recommend that a release agent is used.
• Blue Stuff, good for one off casts due to being reusable, quick from making the pattern to casting the mould, after a few uses it does start to deform.
• 2 part resin, generally good, and will flow in to most areas, though can be very brittle, and fine items can snap.
• Green Stuff needs to be worked in to the mould and not suited for silicon moulds of other flexible moulds, never fully hardens.
• Milliput as above needs to be worked in to the mould and not suited for silicon moulds of other flexible moulds, but does harden, could be come brittle, though with a long working time most flash could be removed before fully hardened.

I think all of these have a place in moulding and casting, but you need to ensure the correct combination.