Replies

1. 
   lat_axe | Aug 07, 2020 03:38am | #1

   If you're sanding out the softer areas (the rapid early growth of an annual ring) between the hard lines of late growth annual rings then your sanding pad/sanding paper must be distorting so that it can "dip" below the hard stuff to sand off the soft stuff. So yes - your sander-to-wood interface is probably too "soft".

   In addition, you may be pressing down too hard on the sander in an effort to speed up the sanding process, which will help a soft sanding pad distort even moree to sand down between the hard part of the annual rings. Let the sander weight do the work, even though it takes longer. (Are you going through all the grits, starting with rough/fast and moving up to smooth/slow)?

   Many low-price sanders put on a sanding pad that's a compromise between hard and soft - or just plain low quality - so it can do this hard-ring-line raising with softwoods having those large (and much softer) early-growth rings. Some sandpapers can be "soft" too - those with thin paper substrates and too much fluff to grab the velcro hooks of the sanding pad.

   The solution is to get a sanding tool fit for purpose and to then use it with good technique. That means a sander with a genuinely hard and flat sanding pad; and sandpaper that will also stay flat rather than distorting to go below the hard part of the annual ring to sand off the soft part.

   Personally I learnt this lesson long ago, via a Black & Decker random orbital sander that was just a TSO (tool shaped object) rather than the sort of tool that does what it's supposed to do. I then went from one extreme to the other and bought a Festool RO sander and sanding discs, which have worked extremely well for 20-odd years.

   Better sanders tend to have accessories to enable various tasks. That Festool, for example, can have a hard, medium or soft pad mounted on it. (The softer ones are good for curved surfaces). The hard pad is flat and keeps the surfaces that are sanded flat, when used properly (i.e always moved over the surface evenly and with no undue user-applied pressure) so it never digs a shallow bowl.

   So perhaps you might get your current sander to work somewhat better than it is to flatten your softwood if you follow the recommended technique for using such sanders - light pressure (or even none, except for the weight of the sander itself) with constant movement over the whole workpiece despite the temptation to sand off obvious bumps rather too rapidly.

   There are well-made sanders other than Festool, of course. But generally, you gets what your pays for. :-)

   Lataxe

2. 
   rob_ss | Aug 07, 2020 07:12am | #2

   Agreeing with @Lataxe here - I had a nice bosch sander which broke - only had a firm pad tho. It was cheaper to buy a new sander than a new pad for it, but I regret that choice as the cheapo sander is horrible.

3. 
   fivebirdscustom | Aug 07, 2020 10:04am | #3

   Time to hand sand with a solid, hard block.

   It's just the way it is with soft woods like pine and redwood. Any orbital sander that has a hook and loop pad, will have too much flex to get a truly flat surface. It can be smooth, but chances are it'll be wavy.

   Since you've hopefully gotten the surface close to where you want it, you shouldn't have to work too hard to finish it off with the hand sanding. But, sometimes hand work is the best answer. I honestly believe this is one of those times.

4. 
   john_c2 | Aug 07, 2020 02:50pm | #4

   Yup. Hard block and hand sanding. Briefly with 120 to flatten, then 220.

5. 
   user-7526330 | Aug 07, 2020 05:18pm | #5

   Agree with hand sanding, and use a large block. If you can hold it, something you can use a whole sheet of paper on. The other issue may be your choice of abrasive. If you can find it, try to track down garnet paper. Quality can be an issue, but it will probably be cheap.
   But you'll be sanding quite a bit to remove the ridge & valley pattern you've already established. Sorry we can't come up with a compelling reason to have to buy a festool sander....😀

6. 
   geedubbee | Aug 08, 2020 12:48am | #6

   Another large hard sanding block fan here. Random orbit sanders will create undulating surfaces even on hardwoods. They often get things “flat enough“ but the greater the difference of hardness in areas on the material the greater the risk. I run very hard pads but, they still are only 5" or 6" in diameter.

   For larger fields, if your not a hand plane kinda guy, the ROS operator needs to exert a lot of control to avoid the small footprint from following the path of least resistance. A large shop made sanding block of MDF with a stiffening rib will allow you to level the surface. Finer abrasives on your ROS after than will cause less undesired results.

   
7. 
   lat_axe | Aug 08, 2020 03:22am | #7

   Some tools are blameworthy. They're badly designed and made, often of poor materials. But in many cases its the user of the tool that's causing an issue by not fully understanding the tool actions or the wood being tooled and then using the tool in a less than optimum fashion.

   All sanding implements are like this, including hard sanding blocks with sandpaper wrapped around them. It's just as easy to over-sand a particular area, often with too much pressure as well, so creating some form of un-flatness or hollows. You can do this with hand sanding blocks as well as with RO sanders. It just takes longer (both to sand anything and to make a dip) than does an RO sander.

   ****
   One way to avoid the dips and hollows is to use a long plane rather than a sander as the implement to achieve flatness and to get rid of what isn't wanted of a surface. A try, fore or jointer plane will be best for flattening. It's also a lot faster than sanding at removing material. It won't prefer early over late wood in the redwood either.

   Sanding is best left as a final finisher - although many will prefer a scraper. Mind, a scraper too can dig hollows - although it's less likely to prefer early wood to late wood.

   On the other hand, a belt sander in a sanding frame might ride on the late wood ridges until they're co-planar with the early wood ..... (That's a guess). A belt sander platen is flat steel - not unlike a hard sanding block only with power. The sanding frame would prevent other sorts of dig-ins.

   *****
   But perhaps the OP would be best to try a longish plane to remove the late wood ridges created by sanding? A very fine cut going with or slightly askew to the grain.

   ***********
   Garnet paper - will take ages and many sheets of the stuff to take significant material. It's a sanding medium best used for final burnishing rather than sanding-off wood.

   Lataxe

8. 
   wood_mouse | Aug 08, 2020 11:57am | #8

   Thank you all for your replies! These are all helpful tips, and Lataxe thanks for the detailed explanations of what's causing the issue.

   I did originally start at 60 grit and go up from there.

   For the orbital sander, I'm not applying any pressure from the top, but it is a bottom-of-the-line Ryobi. I was actually looking at a Bosch orbital sander that has a hard backing pad option but is similarly budget priced. Is this likely to be helpful with the hard pad or not worth it versus going straight to hand sanding?

   I also tried a small 3M rubber sanding block, but probably did not put enough time into it to see a difference. The rubber feels pretty solid to me. Is this type of block generally hard enough? Also, is the benefit of a larger block more about efficiency or assuring that it's flat over a larger area?

   What length counts as a longish plane, and does it matter that I've already filled the knots with epoxy? At the moment I just have a small hand plane.

9. 
   lat_axe | Aug 08, 2020 01:11pm | #9

   An RO with a genuinely hard backing pad that stays flat does make a difference. As another poster notes, though, hook & loop sanding discs - especially if used with an intermediate additional hook & loop "pad saver" - can still make the sanding disc operate as though it was mounted on a softer pad. Light pressure (or none at all) is the answer. Let the sander do the sanding with nothing but it's own weight.

   But, it probably is good advice to use a hand sanding block or something hard and dead flat (a nice chunk of rock maple or similar perhaps) to avoid your redwood ridges, even if it takes rather longer than a machine sander to get where you want to be with your surface. You still need to avoid making a dip with a hand sanding block by sanding too much in one place, although it's very unlikely to sand down into the early grain whilst leaving the late grain wood.

   Glue-back sandpaper on a flat hardwood block is probably the flattest sander you can make. Loose-mounted sandpaper can ruck-up and sometimes tear as well, so the stick-on stuff is better really. Make several with blocks different grits on perhaps?

   A larger hand sanding block can be better at keeping everything flat, as well as a bit faster. With all sanding, it's oh so tempting to apply lots of downward pressure, especially when hand sanding, as it's a slow business. But pressing down hard is actually slower than a light touch, as the grit needs to be allowed to cut and leave the dust behind. Too much pressure traps the dust in the sandpaper so it can't do it's job properly because it begins to skate on the dust rather than cutting more wood particles off.

   The rubber sanding pads may be hard enough to avoid distortion if not pressed on too hard. I've not used one myself. I tend to use an Abranet hand sanding pad of hard plastic with a nozzle to suck the dust out of it via a hose and dust extractor. These work very well as long as you don't press down. They're hook & loop so pressing too hard can distort the sandpaper into slightly not-flat; or distort at the workpiece edges to round them slightly.

   *****
   Longer planes: perhaps they start with a 15 inch long jack plane; or maybe it's really the 17 inch foreplane that's the first long one. The most common long plane is the jointer at 22 inches - to straighten & square glue edges but also to flatten large panels such as bench and tabletops.

   If your pieces to be flattened are fairly small, a jack plane will act like a foreplane would on larger items. The longer sole rides the blade over hollows and just cuts off the bumps, until there are no bumps; and hence no hollows. If the workpiece is small, the jack plane will ride over it's hollows.

   In truth, to rid your surface of those late grain ridges, you could just use a smoothing plane, especially if the workpiece is otherwise flat. Just go carefully, with very fine cuts, until the ridges are all gone. Use pencil marks over the whole workpiece to see where you've removed some wood and where you haven't.

   Lataxe

10. 
    geedubbee | Aug 08, 2020 01:20pm | #10

    Post #7 appears to be SPAM.

11. 
    wood_mouse | Aug 09, 2020 11:59am | #11

    Thanks again, Lataxe! I'll give the plane a try and make some better sanding blocks. And really appreciate the primer on sanding - I was really bearing down on the block before.

12. 
    legoman | Aug 09, 2020 12:57pm | #12

    I personally think bench planes are the way to go -- unless you are making a large table, a jack should have more than enough sole length. I agree that you could probably get away with a carefully-deployed smoother.

    One point no one has mentioned: if you choose to use a plane at this point, you will want to clean the wood of all the sanding swarf very carefully (like with a vacuum or compressed air). Any small particles of abrasive that remain will quickly damage the keen edge of your blade.