Many DIY mechanics often use sealants on the reassembly of their cylinder heads onto the head gaskets and cylinder block. This practice is invariably bad for the head gasket for many reasons and any type of sealant should NEVER be used on head gaskets under any circumstances. This useful article points out the many reasons why you should never use sealants. It is an obvious bad practice and has been known for many years but some continue regardless.

https://ssch.com.au/trade-news/warni...se-of-hylomar/