Hi temp epoxy choice?

Hey guys,
What is the best choice for epoxy for modifying cylinder head port shape? I’ve seen many photos of ports that have been modified with an epoxy of some kind and was wondering if there is something out there specifically designed for that application or if guys just experiment with different stuff to see what works. I imagine it would have to have good chemical resistance and be able to withstand at least 300*F temperatures. I would think a flowing epoxy would be better than knife grade also.
What do you guys use?