I can share a thing or two about Varnish, I've been laying it down for over thirty years.
Especially the last ten years on our Flying Cloud.
Here is a quick primer on my system.
I never use a sand paper rougher then 120 on bare wood, nor finer then 180 grit. The reason is anything below 120 on bare teak will just gouge out the soft grains, that will take forever to fill with varnish. Anything over 180 will generate to much heat and raising the natural oil in the wood leading to bonding issues.
To strip varnish that is well bonded I uses a stripper and saran wrap. Old crazed varnish I use a 1 inch wide red devil scrapper and a classic
cabinet scrapper. Sometimes a heat gun, If I don't have sensitive surfaces nearby.
Now that the wood is scrapped, sanded and fared, comes the the sealer coats. But before that wet out and wipe liberally with turpentine, changing rags often, do this just before the first sealer coat.
Now to the sealer, my recipe is as follows 25% same of the varnish that will be used for topcoats, 60% penetrol, and the balance turpentine. I apply in about 3-4 foot square areas, then I go back and wet sand it into the wood with 180 wet paper. Keep moving in 3-4 sq.ft areas, at the end of the second application area go back to the first section, and wipe the excess sealer diagonal to the grain with a heave burlap. Keep working until all of the first sealer coat is applied.
This method cause all of the soft grains to rise, and the wet sanding knocks em down, the slurry created by the wet sanding fills in the pores in the soft grain.
Now after about two hours or when things are starting to tac up, apply a second coat of sealer, this time mixed 50-50 varnish and penetrol with a splash of turpentine. Lay down a nice even coat. The next day go back and scuff with a green scrubber just enough to knock off any roughness, and and shine. Wipe with turpentine and clean rags. The apply a coat of varnish mixed 75% varnish, and 25% penetrol with a splash of turpentine.
Now you have accomplished 2 things 1. you've created a bonding coat that will move with the wood, and 2. created the perfect tie coat on which you will lay down the rest of your varnish.
Wait about 2-3 days so everything can off gas. Now sand with 320 dry paper just enough to take off the shine, uses a new green scrubber to work any areas that the sand paper didn't dull, vacuum, and wipe with turpentine and clean rags.
Next apply your first coat of varnish use it full strength, and only add enough penetrol to keep the brush from dragging. Next day do the same, the following day do the same. By this time you should have on 3 coats of full strength varnish on.
Now comes time for my secrete weapon. Varnish fails for a number of reasons, movement of the wood, which we solved
with the penetrol sealer coat. UV degradation at the wood to sealer/varnish interface which we will solve next.
Now you're going to varnish the next 2 coats with semi-gloss of the same brand as your gloss topcoat. Using the same sanding and turpentine wipe as before, with just a splash of penetrol. Semi-gloss has a particulate oxide that gives the varnish a semi-gloss appearance. But if you're going to all the work of a proper varnish job it better be gloss.
What these 2 coats of semi-gloss due is act a UV filter, when dry it does not hide the beauty of the wood grain. As long as you don't stop here. What it does do is cause the UV to scatter before it gets a chance to hit the wood to sealer coat interface in a direct fashion.
Now were are going to do 4 coats of high gloss varnish using the same sanding/turpentine regime.
That's all the varnish we will put on for the next 9 months to a year, depending on location.
Next year we want to hit it again with 4- 6 coats of straight varnish, sanding and turpentine, with just enough penetrol to keep the brush from dragging.
Now all you have to do is a couple of refresher coats. If in the tropics you will need do them once a year, if in the Northern latitudes you may only need do them every 3 years.