How many flapper disks should I realistically buy for a 32' x 17' deck?

You are going to burn trough a five-pack of 40-grit flapper disks before you even finish the first 50 square feet. On a 32' x 17' deck, that is over 500 square feet of weathered wood. If you insist on using an angle grinder, you'd realistically need closer to 40 or 50 disks becuase they clog instantly with old gray fibers and any lingering sealer. But honestly, an angle grinder is the wrong tool for a surface this large. You'll end up with deep gouges and a wavy surface that looks worse than the weathered wood does now. Since the wood is already splintered and cracked, sanding it down to fresh grain often reveals that the structural integrity just isn't there anymore. Pressure-treated wood only has the chemical protection so deep, and once you grind off the top layer of a board that's already 'toast,' you're essentially just polishing a sponge. The contrast between your new board and the old ones is annoying, but sanding the old ones won't make them match. Old wood has a diferent cell structure and takes stain much darker than new lumber. If the boards are truly solid and you just want to clean them up, rent a **random orbital floor sander** or a drum sander from a local hardware store. It will save your knees and provide a much more uniform finish than a handheld grinder ever coud. You can knock out that entire square footage in a afternoon rather than spending three days hunched over a grinder. If the wood is as bad as you describe with cracks everywhere, you are better off puting that disk money toward **replacement boards**. Once the wood starts splintering and checking deeply, no amount of sanding stops the rot from the inside. If you can't afford to replace the whole deck, try flipping the boards over. Often the underside is in much better shape and just needs a light cleaning rather than a heavy-duty grind.

I'm realy glad I saw this becasue I was thinking about trying the same thing on my porch. I honestly didn't know you coud actualy flip the boards over—is that a common thing people do? It sounds way easier than sanding. Also, you mentioned the floor sander rentals; do those things require alot of strength to hold onto, or can a total beginner handle it without it flying away? I've never used anything bigger than a little hand sander so I'm a bit nervous about the heavy machinery.

Spot on about the floor sander. I tried the grinder approach on just a small 10x10 section and it was a nightmare. One thing the expert didn't mention is that angle grinders spin so fast they actualy 'glaze' the wood. The friction heat can melt the natural resins or any leftover sealer right back into the grain, which seals the wood fibers shut. If you glaze the surface, your new stain is just going to sit on top and peel off in six months. Renting the big orbital is way safer for the wood's health, and honestly, your lower back will thank you for not crawling around with a grinder all weekend.

If you're dead set on using a grinder, don't waste money on cheap flapper disks that clog in five minutes. Grab a Diamabrush Wood Deck Tool instead. It has diamond-coated blades that don't clog like sandpaper and acutally strips the gray wood off without glazing the surface.

I used one on my 500 sq ft deck last summer and it saved me a fortune in consumables compared to buying 50 flappers. It’s way more aggressive than a 40-grit disk but leaves a cleaner finish if you keep it moving. Just wear a heavy-duty mask becasue the dust it kicks up is no joke.

While the Diamabrush is a superior alternative for material removal, you must meticulously account for the fastener depth across your entire 544-square-foot expanse, as any screw heads protruding even a fraction of a millimeter will instantly shear the diamond coating off your expensive specialized tooling. I woud strongly advise you to spend an afternoon with a countersink bit and a high-torque impact driver before you even consider bringing a high-speed abrasive tool near those boards, beacuse the structural oscillation of an angle grinder at 11,000 RPM creates a harmonic that tends to back out loose fasteners, leading to a catastrophic and costly failure of your sanding media.

Quick update—I’m glad I asked before buying that five-pack. The warning about the wood glazing from the heat relly hit home becuase I tried a small test patch and it looked scorched. I’m looking into that Diamabrush tool now; it seems way better than burning trough fifty disks. Has anyone actualy tried flipping the boards? Some of mine are pretty warped so I’m worried they won’t sit flat.