So I am ready to varnish the ballast of my Regent NH1204M fixture. I bought this varnish but I don't know how I would apply it. If someone knows how to varnish transformers, could you list the steps?

Picture of ballast + varnish >> http://imgur.com/zcZI97u

First you need to warm the ballast up by turning it on and using it. Second you then need to cut power when it reaches the max operating temperature it will reach in the period you leave it on (several hours). Third you then need to apply the varnish immediately after the power is cut, the heat will expand and allow the varnish into the books and crannys of the ballast windings.
Hope this helps.

How do you apply the varnish? Do you use a paint brush or do you pour it on?

Could I use a syringe to apply it?

Yuo can use a brush for the majority of the windings and then s syringe or smaller brush for the hidden areas.

run it hot overnight.
this dries it out.
find a container just big enough to submerge the ballast in with a lid.lid must fit well and seal as you will be containing a vacuum.
you need a source of vacuum and a port on your lid to apply it.submerge ballast in varnish while still hot,close container,and apply vacuum.
wait till bubbles stop and its soaked a while.pull it out,let it drip into container and then pour the remainder of the varnish back into the can.
enjoy your protected and now quiet ballast.

I was looking for instructions myself.

Personally I think the vacuum bit is an unnecessary step as you'll lift up to drain your ballast; what little holes you manage to fill in with a vaccuum will be drained out again. I would also warm up the liquid to say 110F or 40C.

I'm about to use fiberglass resin (the liquid you dip fiberglass cloth, or coat a boat finish) to seal repaired turns. The damaged wires have been soddered. More on this after some results.

Good luck, lights*plus. I think you don't need to vacuum anyway since the ballast already has that done.

Warm the ballast either by running it or heating it in an oven next dunk it in varnish leave it for 5 minutes then pull it out and leave for a few hours to rest