: One reason for sanding between coats is so that you can see the difference
: when you apply the next coat. If you are adding more varnish over an
: already shiny surface it can be really difficult to tell where you have
: been.

You need to follow a pattern the same way on each boat and each coat. Look at the job as a complete kayak that needs spraying and not a coaming, a foredeck, a hatch rim, a hull, etc. Picture the whole job in your mind and you will not miss spots.

Get down close to and at an angle to the surface as you spray and you will easily see the difference between the coat you are spraying now and the previous coat even if they are only being sprayed 5 minutes apart.

: Varnish does not need sanding to get a good bond as long as the
: existing coat is clean and not too old.

Yes, it is like laquer, it is thermal plastic and will reflow with a solvent. This property means it has poor resistance to chemical spills such as gasoline or campstove fuel.

: I have an inexpensive (Wagner) HVLP sprayer which works fairly well. There is
: a learning curve to spraying. I have had a problem with orange peel. This
: can be reduced by thinning, but that can cause sags and drips and slows
: down the drying so dust is a bigger issue.

I have heard that this is a problem with airless sprayers but have never used one. Have you tried applying the material a little thicker without adding thinner? This allows the material to flow out for a longer period after being applied.

If this doesn't work it is a problem with atomization and will only be cured with a different fluid tip and nozzle which may or may not be available for the Wagner. The tips and nozzles are usually sold together and are designed for specific purposes so a varnish will require something different than a latex paint which is probably what your Wagner was designed for.

Ian