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Do you need to apply ridigizer to fiberboard?
If so, how to apply.
Thanks in advance
the other Jim

Rigidizer is great but I've found it attracts glass like flypaper attracts flies.
Question to ask is will it make the mold any better in some way that is worth that risk.

OK, thanks Kevin
I will try the fiberboard.
Will I need to apply a glass separator?

The other Jim.

Oh I'd expect you to have to apply a separator otherwise I think you'd have heard of "this perfect mold material that never sticks to glass and you can use a zillion times" We'd have those specialist separator companies worried.
Everyone's kilns are different and the sources of mold materials vary by country and supplier.
I noted decades ago one particular material I could get in Canada was vastly superior to that which you had in the US but times and suppliers change so I don't know what the current situation is.
You always have to experiment and don't expect the first try to work the way you want it to.
Note I did not ever say not to use rigidizer I only warned about the rigidizer I could get here in Canada attracting glass like flies to flypaper.
What rigidizer does do is make the mold material more brittle for handling but that is my experience. What previously could be a dent can instead be a break of a mold if you are careless.

There are 2 kinds of rigidizer, colloidal silica and colloidal alumina. Silica tends to land on the surface and sticks to glass (often the pink stuff). Alumina is probably what the boards are made with in the first place (green stuff as I’ve seen it). I sinks in and doesn’t stick to glass as much. Boards come as LD MD or HD, light medium or heavy density.

Silica dies if it freezes so it isn’t shipped in winter. Alumina comes as a power you mix with water.