Issues with power sanding

[Duane Bledsoe]

Well, for Christmas my wife and I got the idea to make our daughter a coffee table. I got a stain grade wood shelf (a glue up made from 3" boards basically) from Lowe's to use as the surface, it's a shelf that was 16" across and 48" long. I routed the edges and sanded it with 100 grit paper on my 1/4 sheet palm sander. It was all I had at the time and I thought it would be enough. During the routing of it a couple of the knots in the center actually fell out so I glued them back as best I could and when they were dry I had to sand them flush with the surface again so those spots got more attention than the rest of it did.

When we stained it I could see marks left in the surface from the sander, so, as I should have done from the start, I went back to Lowe's and got some 150 and 220 grits and came home and re-sanded the entire surface with a fresh 100 grit, followed by a 150, and then a 220 to finish. I got a lot of the marks out, but some just would not come out. I can't sand it anymore because the routed edge is starting to thin out in places so I have to live with it now. We stained it fairly dark to help hide the marks and it took care of most of it (I actually thought it was going to make them show worse but it didn't).

My question is, had I started with 100 grit and followed with 150 and 220, over the entire surface, would this have not happened?? I'm also unsure of how fast I need to move the sander as it crosses the board. I sanded in the direction of the woodgrain, not crossways, but I admit I'm bad to change directions in the middle of the board, or swirl it over one spot, as I did in the sanding of the knots to smooth them down. Also when I get to the end of the board and reverse direction again, I basically "hang a loop" and start back the opposite way. The second time I sanded it all down I only allowed myself to pass from end to end of the board in a straight line and come off the board completely before starting back the other way. I have no idea what technique I should be using. All I know is I have marks and do not exactly know what I could do to prevent this in the future. I also recognize that some of what I was doing must be wrong, such as the swirling. So what's the proper way to sand??

[glenn bradley]

I'm no expert but, I have pushed a bit of sandpaper around. Sanding is really the act of applying a layer of finer and finer scratches to a surface until it reaches a state that we call smooth. Stepping sequentially through the grits is key to eliminating sanding marks. If you picture the scratch left by 100 grit magnified a few times, the 150's job is to lay in new scratches that are smaller in depth and width. Then the 180, 220 and so on. If you go from 100 to 220 without stepping in between, the 220 grit cannot overcome the 100 grit scratch size and this is where your marks come from. Also, careful cleaning of the surface between grits prevents your 220 from picking up a leftover chunk of 100 grit and dragging it through your carefully stepped surface .

The difficulty with soft woods with knots is the marked difference in hardness between the knot and the surrounding wood. Power sanding can create a sort of rolling hills effect where a good sized block used carefully by hand will not. I would say that on your pre-made board, I would have left the power sander in the drawer, wrapped some 180 around a flat block, sanded with the grain, blow the surface clean and do it again with 220 and go with that.

[Duane Bledsoe]

What technique do you use with the block?? Back and forth, end to end, gentle pressure or moderate??

[Richard Wolf]

Well, you say you bought stain grade wood "with knots in it". I will assume you got yellow pine shelving, pine is never easy to stain and you really should use a sealer or conditioner on pine before you stain it. Start using better wood, most people want to build things out of pine because it's cheap, the problem is, it is cheap.

[Duane Bledsoe]

I did use a pretreatment before staining. The stain came out beautifully, no splotchy places at all. The sanding was the only issue I had. What I meant by stain grade was that it said that on the packaging the wood was wrapped in, but I know it's not a top grade of wood. I actually like the knots though, it adds a look to it that I really wanted, but mostly I went with those shelving boards because they are wide enough to do what I wanted, and I am fairly new to woodworking so I don't have any experience making my own table tops from glue-ups. Overall, the project is turning out quite nicely for a budget that was limited. I'm in the process now of applying the finishing coats. I have a gallon of Varathane left from when I finished my steps here at home and used it for a durable finish. I figured if it's good enough for a hard wood floor then it will be durable to withstand plates and cups skidding across its surface from eating in front of the TV, and also anything a 2 year old can throw at it as well.

[Ruperto Mendiones]

Two factors I have omitted [therefore learned] that make quite a difference:
1) lighting
2) cleaning off the surface

Overhead fluorescent lighting does not reveal sanding scratches very well. A low raking intense light projected over the sanded surface does.

2) blow off the surface with compressed air or wipe it off with a rag and paint thinner or a tack rag. That way
errant scratches can't hide under sawdust.

Best wishes with the next project.

Ruperto

[Roger Bullock]

A lot of new woodworkers cut it short when it comes to sanding but it is one of the most important steps to a good finish. Use good sandpaper, not sure the box stores carry that good of a grade. I've never had much luck with a palm sander myself and would do it by hand if I had no other choice. If you keep at this you might want to invest in a good random orbital sander, they work well. As far as Varathane, a lot of people will disagree with me but I really like to use it on furniture, especially tables. It is forgiving and has a finish that holds up well for heavy daily use. Hot and cold drinks will not leave a ring like a lot of finishes.

[John Coloccia]

Can you post a picture? It's difficult to diagnose without seeing exactly what you are seeing. Generally, though, if you can feel it, it will be visible in the finish. I find that my fingers are far more reliable than my eyes picking up surface iregularities, especially when the wood is unfinished. It may take a few seconds of "meditating" to get that sense going, but if you just concentrate on it you will find that your fingertips are incredibly sensitive to the slightest bumps and scratches.

[John Coloccia]

BTW, for anyone who thinks I'm nuts, all you have to do to prove this to yourself is to run your fingers across the smoothest thing you own. A piece of glass. Your guitar finish. The mirror in your bathroom. If you let yourself, you will feel slight imperfections that you never knew were there. It's quite annoying after a while, actually, to feel just how imperfect everything really is.

[john lampros]

As someone already mentioned your little palm sander is the culprit. Those little 1/4 sheet vibrators are dinosaurs. They leave millions of 1/4 in circles on your wood. you can get a decent random orbit for less than $50. and you wont see a mark. Also 100 grit is a little strong for pine. I dont even own 100 grit. 120 is as course as I go. you could get by with 120 , 150, 180 and finish with 220. in other words dont skip a grit. With a random orbit you wont have to pay attention to the direction your sanding but out of habit stay with the grain. and on soft woods dont bear down on the sander. you shouldnt anyway but soft wood is especialy unforgiving that way. Your choice to preseal was smart with pine. the best preseal is a "pee coat" of shelack 1 part shelack to 6 parts denatured Alcohol. for hardwoods 8 parts alcohol. presealing gives you more control as to how the wood absorbs your stain and makes for a more even job. less splochy. End grain was almost always presealed on better jobs no mater what the stain or wood was so that it comes out the same depth as the face grain and not go dark on you.

[Duane Bledsoe]

Wow, tons of replies. I can post a picture but I'm not sure it will take that well, except for the largest of the marks. Basically I saw what looked to me like long squiggly patterns, 1/8" or smaller that looked like a stretched out notebook spiral. And in a place or two I saw what is obviously where I moved the sander back and forth in figure 8 patterns over a spot to sand down a raised knot. They are not very noticeable now that the finish is going on but I want to avoid this in the future.

As for lighting, I was sanding outside using daylight. I don't have a shop where I can saw, sand, and otherwise mill wood indoors. I have a basement garage but it would get dust all over the house if I sanded or sawed inside so I just do that stuff outside and then assemble and finish inside. I also did not wipe the board very well. I blew off the dust by mouth, and then wiped it with a dry cloth. That was probably wrong. Lazy on my part, I have an air compressor and blowgun, but also not knowing about using a damp rag with denatured alcohol comes from inexperience. I guess I'll just have to try harder next time.

[Andrew Joiner]

Basically I saw what looked to me like long squiggly patterns, 1/8" or smaller that looked like a stretched out notebook spiral. And in a place or two I saw what is obviously where I moved the sander back and forth in figure 8 patterns over a spot to sand down a raised knot. They are not very noticeable now that the finish is going on but I want to avoid this in the future.

Yes, it's probably your sander. Test it on a scrap with new paper, don't press down at all just guide it back and forth. Look for the marks with good side light. Light sanding with the grain should remove the squiggles . You can use this process or buy a random orbit sander. I have a Milwaukee 6021-21 and it leaves no marks. A great sander for $50.

[John Coloccia]

Funny...I find the ROS to be worse than my finish sander when it comes to leaving visible marks. I use a Porter Cable Speed Bloc, and don't get any marks at all.

[Kevin Godshall]

JMO... I think 100 grit is a bit too aggressive to start off with. Unless you have serious wood to remove initially, or a painted surface you are trying to bare..... 100 is harsh and will leave some deep scratches, especially in a softer wood.

You're glued up piece from Lowe's should have been sanded already to around 150 by the manufacturer. It may have fuzzed up a bit, due to humidity changes, but should be ready for final sanding. Maybe a quick hit with 150 and then onto the finish sanding with the 180, 220.. how ever far you want to go.

I've spent a lot of time, sanding out sander marks from being too aggressive with my sanding in the past. Actually, it wasn't really sanding, I guess you would call it "shaping with sandpaper". Either way, the grit in 80 and 100 grades is very coarse and will make deep scratches. Only way to remove a scratch is to sand below the depth of the previously made one.

Hope this helps.

[Forrest Bonner]

+1 on using better wood to begin with and not starting with 100 grit sandpaper. After you have 'finished' sanding with your highest grit, try wiping the surface with alcohol or mineral spirits (depending on what finish you will use) and you should be able to see any sanding defects when wet.
Forrest