The Fine Points of Scuff Sanding

"Scuff sanding" to knock down minor imperfections between coats may be done quickest and best by hand. Here, professionals delve into the fine points of how, when, and why to scuff sand finish coats. August 10, 2005

I am wondering what everyone uses for between-coat scuff sanding. We have been using Porter Cable model 330 finishing sanders, but they seem to break down way ahead of schedule. Has anyone found a good finishing sander that works well for scuff sanding and lasts longer than the 90 day warranty? We use Dynabrade DA's for general sanding, but they are a little too aggressive for between-coat scuff sanding as they burn through around the edges far to quickly. What are all of you doing in your shop? Any help is appreciated.

Forum Responses
(Finishing Forum)
From contributor M:
I'm from the old school, but scuff sanding used to mean to lightly sand by hand to remove defects in the coatings. One hand would feel the surface for any nibs, dust, or dry resin particles, while the other hand would sandpaper out those imperfections. Maybe, you don't need a new sander after all.

From contributor R:
You should be able to turn down the speed on the DA and sand right up to the edge without burn-through. If the machine is in good condition and well maintained, there should be no problems with swirls.

From the original questioner:
Here lies the problem. We do way too much volume to sand by hand, although that may very well be the correct or best way to do it. Second, I am not the one doing the sanding as I am the owner, salesman, designer, builder, bookkeeper, and a host of other things including responsible party when things go wrong.

The finisher and helper do most of the scuff sanding, and I try to instruct them in the best way to do it, however I am not able to be in that area all the time and I need methods that are easy to carry out. It seems that when they have used the DA type sanders that when they get near an edge they sometimes get a burn-through and lighten the stain which is then requiring a touch up.

Perhaps slowing the sander down will accomplish this? If not, then I am back to looking for a sander that will stand up to industrial use.

From Paul Snyder, forum technical advisor:
For sanding primer, I use a random orbit sander. I switched to Festool over a year ago and it's still running strong (though I have had one repair under warranty). It sure beats getting a new sander every 3-4 months. For scuff sanding, I like the sanding sponges 3M makes. The fine sponge is a little coarse, but comes in handy at times. The superfine is just right for nib removal and light scuffing.

From contributor C:
For a scuff sanding of the sealer coat I use a 3M sponge block or a Scothbrite pad, and Iím not sure that I see how using a sander would speed things up. If you're talking about a glass smooth finish complete with filling pores, then I can see using a machine.

I like the hands on approach and sand to touch. I can scuff sand a raised panel door faster than a guy could run an orbital over it, and I won't burn through the sealer and stain. Not counting filling nail holes or fixing imperfections, it takes me less than a minute for the average sized cabinet door. Plus I'm not being subjected to carpal tunnel from my wrist being vibrated all day long.

From the original questioner:
I think maybe I should have explained one thing a little better. We sand all of our raised panel doors by hand as do most of you. What I was referring to was the main panels of the cabinet. We do all of our finishing flat before assembly and then dowel everything together so all of our parts are being sanded flat as pieces, rather than a completed assembly.

With flat sanding of these large panels, we find that we get a better final finish with the vibrating sanders as we are leveling the finish more than just knocking it down. Much of the cabinets we are doing are furniture as opposed to kitchen cabinets, so the finish needs to be a little smoother than we might be able to get by with kitchen cabinets. We presently use two sealer coats of ML Campbell Magnasand, and two topcoats of Magnamax.

From contributor H:
I use whatever electric orbital sanders I can find, as long as the price and quality are good. I have a quarter sheet PC and a Dewalt, and a powerful Milwaukee half sheet. I would suggest trying the Milwaukee as it's a solid production machine. Also, I would suggest using 3M 216u sandpaper Ė itís excellent no-loading paper and itís compatible with water based finishes.

From contributor B:
To the original questioner: What type of sprayer are you using to apply these coats? I donít see the need of mechanical sanding if the first application is heavy enough. I would suggest just hand sanding to knock down any nibs and such, and apply a heavy last coat.

From contributor T:
We use orbits with variable speed on lowest setting for flats and large panels and hand sand doors, profiles, and etc. I like the way the sanders level out large flat surfaces. We do go back over them lightly by hand to remove any sanding marks and to feel how the finish is ending up.

From contributor J:
To the original questioner: Did you get an Ionix blowgun? Has it helped?

From the original questioner:
Yes we did get one of the Ionix blowguns and it does seem to help. I have noticed considerably less airborne dust getting in the finish.

From contributor N:
Scuff sanding is a light sanding to aid the topcoat in adhering to the substrate. Here's a tip. Remove the orbital pad of the machine and attach a wood handle to it through the three screw holes and then you will have yourself a quick changing flat surfaced block using the same paper as before.

From contributor M:
If someone was sanding on evaporative coatings, no sanding would be needed for these types of coatings because they dissolve when they are recoated, so sanding would not matter.

Reactive coatings need to be scuff sanded mainly to remove any gloss on the coating surface, so you get a better chemical bond, not from the depth of the sandpaper scratches, but from the chemicals in the coatings. These coatings do not resolvate, they will bond chemically as they cure.

From contributor N:
The key is in which sandpaper to choose, but you can hardly go wrong with 220 or 320 grit. The even better key is using the right kind of coating and applying it in double coats. The first one goes on fairly light to grab and fill while the second coat goes on to fill and level. The pre-cats dried fairly hard, and if you tried light coating or faster passes to avoid runs or sags you'd wind up with overspray from the coating flashing off too fast.

The only thing that would return this problem back to normal would be a scuff-sand to help topcoats grab without running or sagging out. The same thing holds true for post catalyzed coatings. If youíre not scuffing up and youíre laying down passes to keep the wet edge, you will find gravity making your day long.

From contributor R:
The equation for sanding reactive coatings involves time as well as grit. If you sand and re-coat a reactive coating within a couple of hours of application, you will get a chemical burn-in of the second coat. After 24 hours there is no chemical burn-in, only a mechanical bond. Most good catalyzed finishes can't be re-softened by their own solvent after a certain point in their cure time.

Cure time in any coating also translates into shrinkage. If you sand a coat while it is still relatively fresh, it will continue to shrink as time goes on, so the sanding scratch under the second coat will also reduce in size.

The older the finish is, the less it will continue to shrink and the more the scratches will stay the same size. So you will probably have less problems with sanding scratches telegraphing if you sand sooner rather than later, and you also get the advantage of the chemical bond. You can also think of this in terms of being able to sand with a courser grit while the finish is fresh and a finer grit as the finish ages.

One last thing - the grading standard used to size the particles on the abrasive can have an effect as well. CAMI grades allow a wider range of particle sizes than P grade. Micron grade allows the narrowest range of sizes. The more consistent the particle size, the more consistent the scratch.

From contributor M:
Regardless if itís on new coatings or repairs, I agree with the time factor about burn-in (chemical bond). When it comes to mechanical bonds I personally don't think that I would depend on it to work if the coating was cured, nor would I need it, if the coating was still fresh.

Scratches or dents in the reactive coatings need to either be sanding out with finer and finer scratch patterns and then be compounded if it is glossed. Whereas, the dents must be filled in either with clears or colors, and in some cases these fills may require both color and then clears. These also must be sanded level, and then sanded with finer and finer scratch patterns to increase the smoothness which in turn will increase the gloss. Re-coating after the repairs are completed usually produce the best repairs.