Fred Williams wrote:
I've never seen weeds on greens, even on the
cheapest of golf courses.

I have seen weeds and moss on greens.

So does that mean
that the grass used for greens repels weeds
or is it because of the skills of green-keepers?

Yes, they use chemical treatment.

Plants are competing against each other. Each sort of plant has its own
optimal conditions as a combination of light, temperature, moisture and
minerals. Some plants like the same conditions as grass used for lawn.
And they survive daily cutting, because their leaves lay flat on the
ground. Here in Europe especially Daisy and Broad-leafed Plantain are
strong competitors - they lay their broad leaves over the grass covering
it, keeping away the light from the grass.

The reason I ask, is because I would like to
make part of my lawn suitable for putting on.

Would overseeding with green-type seed do,

No, new seeds have nearly no chance against the established plants.
Also, I doubt that the existing sort of grass is usable for a putting
green. You can try to cut it daily short, but it will get brown and die,
or it will loose competition against mosses (if moisty).

or
do I have to clear a patch down to the earth (no
big deal) and start afresh?

You need to exchange the soil 1-2 feet (30-60 cm) deep, seed a special
sort of grass, and maintain this area like a green-keeper.

For some medium quality you can try
- break up the soil 1-2 feet deep
- remove all roots of plants and all stones by sieving
- order some trucks of washed sand, mix the sand with the soil
- let the remaining weeds grow out for 1-2 weeks,
break up and sieve again
- form the surface of the green, it should be higher than the
surrounding area, and it should be falling down to let water
flow away.
- seed the grass, best time in Europe is May (warm and moisty)

Helmut Wollmersdorfer