I have been converted to the use of Wellseal, at least one of the cherished repairers uses it and I have done so since observing its use then. I have copied the sales blurb for info, I have no connection other than as a satisfied customer.

Wellseal was developed by Rolls Royce Ltd and manufactured under license, this compound is the complete answer to sealing problems on machined faces.

Stag Wellseal jointing compound is non-flammable, has easy partability and is non-hardening it is formulated to give the best possible sealing performance under a wide variety of conditions.

It can be used to seal threaded connections as well as flat- faced joints and can be used with or without a gasket, depending upon the design of the assembly.

Stag Wellseal Jointing Compound is highly resistant to the majority of commonly used field, lubricants and coolants and is non-corrosive and free from abrasive fillers.

Location: Cotswolds

Location: sunnyish suffolk coast

I have used grease only once. Head and block were machined and stuck together like slip gauges when smeared with oil. Once all bolted up and when filled with coolant it leaked like a sieve in to cylinders and valve chambers! Never again. Hats off to those who can do it but wellseal for me every time now

Location: Farnham

I've always used a smear of grease, never a sealant, never a problem.
You pays yer money and takes yer choice.

Location: Melrose, Scottish Borders

I used to use a smear of Blue Hylomar or Grease , but like others on here I now always use Wellseal (Also known as STAG) and have done for 10 years now, with no issues apart from it sticks to everything WD40 does seem to be the only thing that removes it for clean up.

Location: NSW Australia

The problem with sealants is the gasket may not be reusable. Books such as Nicholson assumed reuse. Recommendations for cyl heads have varied over the years but in the 1960s when car heads were still commonly removed a very thin smear of grease was the fashion, and I complied. My Seven benefitted from decokes and often (using the valves commonly supplied at the time) needed a valve grind after 5,000 miles. I always reused gaskets. Similarly I ran Javelin for over 20 years and for a myriad reasons this required innumerable head removals. I just used a trace of plain grease. No gasket failures and no obvious leaks.And no drama removing the heads.
Gaskets with metal on only one face generally cannot be reused so a sealant more appropriate.
I have dismantled cars where graphite grease has been applied so thick it pressed hollows in the gasket!

Bob Culver

Location: Auckland, NZ