Detecting Sanding Scratches

Advice on lighting a workpiece so you can see fine sanding scratches before you finish. June 30, 2009

I just made a solid oak table with glued up strips. I planed and scraped joints already, starting with 100 grit paper on a hand held belt sander and working my way up to 320 grit. What is the trick you gurus use so see scratches and imperfections before the staining process?

Forum Responses
(Finishing Forum)
From contributor G:
I see no good coming from the use of a (hand) belt sander. This will distress the wood and you will see it in the finish, especially if you use a finish higher than satin. If you are using it to get the boards evened off that is fine. But you will need to use a random orbit sander. After leveling off the boards you can use the same grit to start on the random orbit sander. Then go up to 150-220 depending on what type of finish you plan on using.

From the original questioner:
Thanks contributor G but I never put in the fact of the orbit sander. How do you highlight the scratches before staining?

From contributor B:
I am not a finisher, but what I do sometimes is wet it down to raise the grain, and that can show if you have scratches in it, and then just go over it with your finish grit.

From contributor G:
A few ways, but with oak it will be difficult unless they are severe. If you have a point source light, incandescent, halogen, not fluorescent put it at a very low angle to the top and this will highlight the scratches. The other way is to use a solvent such as paint thinner, lacquer thinner or such and spread it on the surface. This might highlight the scratches, use with the low angle lighting. Use the solvent that you would thin whatever finish you plan on using so there is no chance of cross contamination. Using water may cause problems and will cause the grain to raise.

From contributor W:
The easy answer is to stain the top and if it has scratches sand them out and re-stain. I do it all the time. Of course if you’re using Minwax stain you have to re-sand the whole surface and start over. I use Mohawk stains that allows me to sand a spot and re-stain just that spot and blend it in.

From contributor T:
I use a Halogen lamp mounted in a box with an open back and a top that slopes down toward the front so that the light washes across the work only. Dimm the rest of your lighting, and you'll see the very smallest scratches and swirls. Once I start staining I don't care to stop and back up.

From contributor A:
Wash it with a rag soaked in denatured alcohol. This will pop the grain and help you to see the cross grain scratches. Also, if you purchase your Sandind belts at a hardware store or box store, you are only hurting yourself. Sandpaper is graded either US or Euro. US is 60% of the grit is actually the size on the box (40% are finer or coarser). The Euro graded products are 95% of the documented grit. I rarely use belt sanders, because they can leave awful cross grain scratches that are difficult if not impossible to sand out.

From the original questioner:
Which one do you recommend? I use 3-M whenever available.

From contributor A:
3-M has two distinct lines of abrasives - the typical hardware store variety and the professional. The typical stuff was graded like the Norton. The pro products are graded on the Euro standard. 3-M makes excellent pro abrasives, however they are usually expensive.