Varnish Or Poly over Shellac...

Obviously you are referring to traditional shellac, not the de-waxed variety.

Shellac.net is a great resource for everything shellac.

www.shellac.net/

Thanks for the link RinTin. I highly suggest that anyone wanting to put varnish or poly over shellac use a De-Waxed shellac LOL. Although I still  don't know why someone would want to do such a thing.

Mix-your-own shellac is the way to go here. De-waxed shellac is available in flakes and mixed with denatured alcohol, makes an ideal undercoat for many more weather resistant outer finishes.
As Turbodaddy says, the sanding sealer works great under varnishes and can easily be tinted with alcohol soluable powder dyes.
Mix-your-owns also have a longer shelf life (while still in the dry form) than canned shellacs which go bad in an alarmingly short time, especially once the can has been opened.

I was a custom cabinet maker and countertop fabricator for about 10 years. Struggled to make much of a living at it so I moved on and became a dealer instead. Shellac is new to me as I primarily sprayed catalyzed lacquers and conversion varnishes, but I am no stranger to wood finishing.

In my experience, one problem with water based finishes is they don't bring out the depth and character of the wood very well at all, leaving it looking flat and lifeless. My wife (she has asthma) and I recently re-finished an old mahogany Duncan Phyfe dining table with what was supposed to be a top quality water based finish recommended by our local professional paint store. We are very disappointed in the results. This table should have just popped and instead it looks like poop. I have since learned that using a de-waxed shellac sanding sealer/base coat helps, but after this experience I would need to see an example firsthand before I ever consider a water based finish again.

Another thing I don't like about water based finishes is that the wood underneath it doesn't oxidize and naturally change color like it does with more traditional finishes. This is especially noticeable in woods like Cherry and Fir that dramatically darken as they age.

If you are thinking about trying something different, a product I can highly recommend is Sikkens Cetol Door and Window finish.

www.perfectwoodstains.com/view-product/cetol-door-window?category=doors-windows

It is a slower drying, solvent based product that lays down and flows out very nicely. I have only brush applied it, using a medium stiff natural bristle brush, on a set of VG Fir exterior doors. With finishing, technique is just as important as product selection. These doors were brushed laying flat. After applying, the finish flowed out and the brush strokes totally disappeared. It dried to a very nice sheen, super smooth to the touch and really brought out the character of the wood. Because it is solvent based, it off gasses for a few weeks but the smell is nothing compared to the 2-part finishes I used to spray.

Thanks again guys. You confirm my limited experience.

I only finish a couple of trailer interiors per year, so it's not that big of a deal to me on the possible time savings for all of the miracle new products. I'm sure if I were into a production environment, I might feel differently. I like using the French Polish method for applying the shellac because I can toss the wiping cloth into the shellac can, and only do as many pieces and coats as I want at a time. There's virtually no clean up, or product loss.

Just last night I put the final coat of shellac on the walls of my Aloha. I used the Zinsser traditional shellac that everyone else here is using, three coats of amber and two coats of clear. This was my first experience with the stuff, I wanted to do it by hand to get a good feel for the product and I must say, it was a real struggle for me.

I coated the entire walls at once and for the life of me, I could not get the overlaps to my satisfaction. I too tried the French Polish method and each time I added shellac to my pad, I just could not get it to blend well with the previous application. I tried everything, putting it on from super thin to heavy, varying the amount and type of oil on the pad, working fast trying to keep a wet edge. I also went out and bought a very nice fine natural bristle brush and tried every brushing technique I could think of.

My problem was this stuff gets tacky so fast that the overlaps build up instead of blending in and when I pulled back into the wet edge and lifted the rag or brush it would lift up on the shellac as well, and it wouldn't lay back down. While doing the walls, I also did a small 16" x 30" piece of plywood as a control sample. That piece turned out great as I could coat the entire thing with one wiping. I wish I could say that things improved as I went along but it was just more of the same.

In the end I sprayed the final coat, which is how I will be applying it next time. To look at this picture you might think it looks great but it is not near the quality that I am used to doing. The picture has a lot of reflection but if you look closely you can see the darker overlap areas running horizontally. I am not giving up, but man, this is a very different product than what I am used to.