I'm making some ornamental exposed aggregate concrete blocks to match
those in our 80 year old garden wall.
I'm very pleased with my wooden mould - designed to make 10 blocks at
once.
So that it doesn't get wrecked by the water and concrete, and remains
reusable, I've varnished it with polyurethene, and lined it with blue
damp proof membrane.

Would polyurethene varnish on its own protect the wood?
The dpm plastic is a pain - it flaps about and makes it hard so see
where the screws are going when assembling the mould, and if it gets
creased, it shows in the blocks. It has the advantage though of
allowing easy release from the mould.

Any ideas on a better way to water-protect the formwork?

Secondly, I think there is such a thing as a washer with teeth around
the edges, which I could nail over a screw hole so as to ensure that
the screw can only go in the correct place.
Anyone know what these things are called so I can tell the store what
I want to buy?

Thanks

tony