1980s Eclipse Countryman and Bob Yak

Very nice, and duly noted for metal mudguard upgrades! What if anything did you use to mask it?

I think the idea originally came from my reading Tony Oliver's Touring Bikes book a long time ago, but I don't have my copy in the UK to check.

Here is a somewhat detailed summary of what I did to mask it:
I used 35mm wide STUK masking tape cut into short lengths of about 25mm. I applied each strip with the short side halfway inside a section of the inner lip and wrapped them laterally round outside of the mudguard toward the centre line. Each small section butted against the next along the inner lip. The front and rear of the mudguard used narrower strips. Once the inner lip was completely masked I applied 60--80mm lengths longitudinally to the outside of the mudguard until no gaps remained. After a couple of days drying, the excess rubber underseal came away nicely at the edge of the masking tape, carefully removed one lateral strip at a time.