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I see many, many threads about fixing scratches in poly. And the directions on most finishes is to sand with fine (220) before applying the next coat of finish.

How do I not leave scratches with the 220? And so then having to revert to 120, 180, 220 again ect. I see everything saying just lightly sand before going on to the next coat but I am always getting scratches.

Am I not waiting long enough for it to cure? I would really like to do what I see people do and apply one coat, lightly sand, then proceed to the next coat ect.

Thanks a bunch for any help!

2 Answers 2

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How do I not leave scratches with the 220?

Excuse the double negative, but you don't not leave scratches with sandpaper of any grit. The whole point of sanding is to make scratches. If you're sanding between coats, you might do some scuff sanding to make scratches so that there's more surface area for the next coat to grab onto.

If you're sanding something smooth it, working from coarse to fine grit lets you remove large scratches (or other surface imperfections) by making many smaller scratches, and then replacing those with even smaller scratches, etc.

And so then having to revert to 120, 180, 220 again ect. I see everything saying just lightly sand before going on to the next coat but I am always getting scratches.

Again, the point of sanding between coats is to make the surface rough enough that the next coat will adhere better. You shouldn't need to "revert" to larger grit to remove any scratches made by the 220, and the finish will fill in the scratches from the 220. If you do go back to larger grits, you'll likely remove more of the first coat of finish than you probably want to.

Am I not waiting long enough for it to cure?

I don't know, because you haven't told us how long you're waiting, or what the surface looks and feels like when you start sanding, or what type of polyurethane you're applying.

In my experience, for either water-based or oil-based polyurethane, the surface should feel dry to the touch before you proceed to the next step. Water-based polyurethane dries faster than oil-based. Check the directions for the product you're using to find out how long to wait between coats (should be at least a few hours), and pay attention to temperature and humidity recommendations. If you continue to have trouble, make sure that your product is fresh (old finishes can take longer to cure, or may not fully cure at all), and try increasing the waiting time. Also, you generally shouldn't apply more than two coats in a single day.

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How do I not leave scratches with the 220?

You can't, scratching is what sanding does.

Everyone you see sanding between coats of finish IS scratching the surface, the scratches are just much more obvious in person than viewed on video or in smallish photos in a magazine or book.

So how are they getting away with it? It's simple, the scratches don't matter because the next coat of finish fills them in.

And there's even better news.....

You basically don't need to sand between coats
Despite what the instructions might say it's not needed to ensure adhesion of the next coat, except perhaps if life got in the way and there's been an inordinately long time since the last coat went on.

The one reason you might commonly need to abrade the preceding coat is to de-nib it, which is generally just dealing with a few minor specks of embedded dust. This doesn't require sandpaper, it can be done effectively using nothing more sophisticated than the rough side of brown paper!

If you do need to abrade intermediate coats of poly a little more because e.g. there's some raised grain (should only be a problem after the first coat), or a larger than typical bit of dust landed on the wet varnish, you can sand with 220 but the emphasis should be on lightly — use just the weight of your arm, nothing more.

Some woodworkers do prefer to use 280-320 (and occasionally even finer) and obviously with higher grits the scratching is finer and less evident. But remember, the scratches from light sanding don't matter.

You might benefit from having a look at this on Popular Woodworking, The 7 Myths of Polyurethane by Bob Flexner.

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  • I read these right after you two answered and never replied. Thank you both, these answers helped me a lot. I've been woodworking for about 4-5 months and the learning curve is real! Thanks again.
    – Tim
    Commented Feb 9, 2023 at 14:47
  • "the learning curve is real!" Oh yes indeed! Just wait until you wade into the deep and wide waters of sharpening LOL Seriously though, sharpening may be the most hotly debated topic in woodworking, and it's because people have so much difficulty separating person preference from actual results. Finishing is a little like that, but the subject is more complex; so, do yourself a huge favour early on and head on over to Archive.org and look at the free-to-read finishing books over there. You'll find a cornerstone book from Bob Flexner, who may be the most reliable single source. [contd]
    – Graphus
    Commented Feb 10, 2023 at 8:03
  • There are other good books on finishing, especially by Michael Dresdner (also Jeff Jewitt, Sam Allen, Nick Engler and Joe Lerario) but it would do no harm to read everything through the lens of what Flexner says, because he's done so much to simplify understanding of modern finishing, dispelling myths (like having to sand between coats of varnish), and popularising wipe-on varnish which is arguably THE most versatile way to finish wood — and his writings on the subject are literally the best you'll read; virtually everyone else doesn't explain it as fully as Flexner does.
    – Graphus
    Commented Feb 10, 2023 at 8:19

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