On Wednesday, 5 February 2020 08:49:17 UTC, harry wrote:

The Hammerite is rubber based. I'd put the varnish on first.

The MSDS for brushable Hammerite says nothing about any relation to rubber.

The product description is: Styrenated oil modified alkyd containing organic, inorganic pigment, aliphatic and aromatic solvents.

https://www.farnell.com/datasheets/45976.pdf

I would have expected any rubber-based product to mention the fact in the MSDS due to the issues of latex allergy.