I have a LOT of wood on my boat, and I keep it varnished. If I followed all the techniques listed here, I wouldn't get half of it done.
You have to have a bit of a playful attitude.....really.....keep experimenting and see what works for you.
Heat gun or Jabsco paint
remover work for me if I want to go down to bare wood. But I rarely do. Instead I use curved cabinet scrapers to remove what needs to be removed. Feather it in by sanding
the area. See how it's going to look by wiping with thinner. If it looks OK, then you're good to go. If not, sand or scrape some more.
Look at beautiful, old, wooden, varnished boats whenever you can. You'll be surprised. Their varnish isn't perfect.
Go to Lin and Larry Pardey's website and read Lin's take on patching varnish.
I'm blessed with my boat, that no one piece of wood is very large. So I can isolate a spot to strip if necessary, and if it ends up looking a bit different, that's OK.
As for thinning.....I use Captain's varnish by Pettit, which claims not to need thinning, but as I proceed, it gets thicker, and when it reaches honey consistency, it's too thick for me. Thin with the recommended thinner, despite the cost, it does make a difference in how it flows.
Keep it up! It's worth it.
Note: left photo
shows varnish on left, Cetol on right. No comparison as far as I'm concerned!