On Sunday, 25 October 2020 at 12:26:41 UTC, Dave Liquorice wrote:

What are the washers made of and how soft are they? If they are some
form of thin (1 mm ish) soft rubber they'll do the job "dry" and
without needing to be murdered up. If they are fairly regid plastic
they'll probably need a sealant but I wouldn't use silicone, just
plumbers mait.

The underneath one is soft rubber, and fits/moulds well with the underneath parts and is ridged to presumably seal well with the sink. The upper one is a thin foam; probably ~1mm as you say.

I'm also loath to use a washer on the top between the waste flange
and sink as the washer thickess can create a ridge that holds
water/muck. Really depends how well the flange matches and sits in
the sink. Without a washer I'd seal with plumbers mait.

I've tried with and without the foam washer and I think I'm leaning towards it being better with. There's quite a recess in the sink and so it doesn't sit proud, and if anything it seems to be helping it self-centre.

As I mentioned to newshound I think I'm going to go for dry fit and only go for silicone/PM should the need arise.