In article ,
"m. scott veach" writes:
I'm a stone-cold amateur at woodworking. Doing my very first project
of any kind: making a simple desk out of some plywood and electrical
conduits for legs.

I am sanding the plywood (birch) and I realized that I don't know when
I am done...

I've made a couple of desks in the past. The first one wasn't smooth enough.
When you write on it, the grain shows thru to the paper. So I learn from that
to use a piece of paper and a #2 pencil to check for smoothness. Put the
paper on the desk and, using the side of the lead, shade an area of the paper.
If it comes out evenly shaded, you're done. If the wood grain is obvious, sand
some more.

And yes, the polyurathane will smooth things out if you can avoid runs and
drips. But a smooth starting surface helps a lot and requires fewer layers.

And as others have mentioned, be careful about sanding thru the plywood.

--

Mike McDonald