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I've got a bunch of recycled hardwood flooring that I plan to use for shop projects and possibly nicer furniture.
How should I remove the varnish? Would it be preferable to use the thickness planer or rip it with the table saw? Or is it worth waiting to get a jointer?
I don't want to dull my planer blades when a carbide-toothed saw blade would hold up better, or vice versa.
You can either mechanically remove the varnish or chemically strip it.
Examples of mechanical methods:
If I was using one of the above methods I'd probably opt for the belt sander.
If using a planer or jointer, some people prefer to swap in a dull set of knives. Similarly, you could swap in a cheaper blade if you use the table saw and don't want to accelerate the wear on a nice blade.
You can also often chemically strip old varnish by using an appropriate solvent, as discussed in How to find out what kind of finish is on a piece of furniture, or by using a chemical stripper as discussed in an article on Bob Vila's website.
In smaller batches I recommend hand scraping for finish removal on large, flat expanses as you're faced with here. It's not as onerous as it sounds because of how efficient card scrapers are, and at least you're not having to deal with the boards in situ on the floor. (Backbreaking work!)
Chemical stripping is obviously one option, and while the 'green' products (which are slower but can still work well) are much less odoriferous the cost does rise and rise the more you have to do. For furniture-scale projects I'm a big fan of strippers, but they're a little hard to justify for very large amounts of stuff if you're buying your stripper retail.
Heat or flame stripping is an option. Using a heat gun or blowtorch, with paint scrapers in a scraping motion or a stout putty knife in a pushing motion to scrape off the softened finish. But ideally you'd want to work outside, especially if using a torch.
There's a really good alternative to any of the above that can sometimes work to your advantage: use the back of the board which doesn't have any finish on it.
I've used a heat gun and a steel brush (lightly applied) to get varnish of old furniture in the past.
Depending how much you have might be worth it to get a cheap hand planer to remove the finish before running it through your planner. I would check really carefully to make sure there isn't any metal in the boards.
