Is there any paste available which can be used to fill in pits and cavities in iron castings before powder coating which can withstand the baking temperature of around 140deg C?
Nadi
There are metal filled epoxies available at industrial supply houses. Devcon is one brand, although most of their formulations exhibit temperature resistance a little bit below (i.e. 120C) of your 140C baking temperature. There is a titanium filled Devcon that has higher resistance,however.
Scotchweld 2214.
This comes as a B stage (one part, no mixing). Some of the variations will work up to 350-400F.
You will need to cure these before you try to coat.
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