[retailer]

You don't need a margin if using a bobbin, the bobbin cheeks support the sides of the winding and also provide insulation between the winding and the core. Winding a transformer by hand with a tube instead of a bobbin is do-able but not easy, if you are starting out winding transformers just stick to bobbins.

Some may be able to determine the core material just by looking, I can't, the brown colour you refer to may be a thin layer of potting varnish, if repurposing a transformer the varnish should be removed, don't sand or scrape it off as you will most probably destroy the thin insulating layer on the laminations, use solvent, it is a smelly and pleasant job, leaving it there will affect the stacking factor and also make it difficult to get all of the laminations back in, this is important if you are rewinding a transformer that is part of a vintage amplifier or similar as you really need to rebuild the transformer so it is identical to the original, if it is a new wind on a salvaged core and you don't remove the varnish the transformer will still work but may lose some efficiency.