Reddit reviews How to Paint Like the Old Masters

They often didn't; you can't always tell that from just pictures on the internet, though, you need a really good quality reproduction or just head to a museum and have a look at them up close!

If you just want brushstrokes to be unnoticeable, use a medium that levels the paint, like Galkyd or Liquin. You can still work in opaque layers but your brushstrokes won't be obvious unless you really get in to get a close look.

If you want them to be essentially invisible, use more medium and glaze layers of color over a grisaille base. (Which would drive me insane trying to replicate, I think, because you have to wait at least a day between each layer for it to completely dry.)

This book has a ton of information on the subject, though I'd recommend skipping over the part about making your own mediums unless you're a painting history buff and just look for modern equivalents.

I own the book How to Paint Like the Old Masters by Joseph Sheppard, and he specifically uses the color over grey scale technique. It's an extremely effective approach.