unclear clear satin varnish

IME very few varnishes are completely clear. The clearest would be one of the water based ones. Easy to use and easy to clean up but a real bugger to get a top class finish from but then I am very fussy.

Most wood varnishes are not pure enough to be water clear - they don't normally need to be as a slight yellow tint isn't noticeable on wood.

Yes. You want a more refined one that is truly water clear. The sorts of overcoating varnish used in the automotive industry base coat clear paint technology are an example.

TBH I prefer a slight yellow tint in my wood varnish it makes it easier to see where it has already been brushed out. My yatch varnish has a slightly evil green colour cast from the (nasty) antifungal components.

The only clear varnish I know of is the 2 part stuff from Rustins (Liquid Plastic?). Most urethanes are yellow and d-i-y acrylics are dull and cloudy. Automotive finishes are applied in a temperature controlled environment, which makes truly clear finishes possible.

Hmm. The acrylic varnish we used last time was clear, but not mildew resistant in the long term. I guess I could try that again, & just expect to recoat & sometimes repaint periodically.

OK. I wondered if the problem was using varnish that might be a bit old. I'll try another test patch with acrylic varnish (that worked before on white-stained wood).

I recently got some clear gloss varnish from Wilkinsons and was surprised to find that it was completely clear when dry, I had expected it to be a bit yellowy. It's called "Wilko Quick Dry Varnish (for internal wood)"

That is a. Water based varnish. As I said before water based varnishes are amongst the clearest available

Oil-based varnishes tend to become yellower over time even if they start out pretty clear. Don't think I would expect any to remain water-clear even if they started out that way.