Light sanding Polyurethane leaves visible scratches
I am applying Minwax fast drying Polyurethane to my wood project and very lightly sanding with 320 grit sandpaper with a sanding block between coats (just finished 3rd coat)and it is causing visible scratches in the finish. I want to get the finish a little smoother but I’m afraid if I apply more pressure I will inflict more scratches in the finish. Is this normal to occur, and should I switch to a 400 or 500 grit sand paper to level out these scratches? I am vacuuming up the dust between coats and only sanding in one direction across the entire length. Thanks in advance.
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I use the wipe on poly from MinWax. I use a light touch with 220 between the first few coats. After four coats, I wet sand with 320, again with a light tough, only going over a area with one pass. After the sixth coat, I just go over with 600 very lightly. I apply very thin layers of the wipe on poly, always with the grain. The latest project were two hickory coffee tables. There is some grain with hickory so you will never get a mirror finish unless you do a fill. Works good for me.
Now, just about everyone here will have a favorite method to apply finish. It will be interesting to see other comments.
Interesting. Would you expect to see visible scratches in the finish when lightly going over the surface with 320? The scratches seem to get filled in/disappear when I apply the next coat of Poly over the previous one so maybe I shouldnt worry about it. Or is it acceptable to use 320, and then 400-500 between coats? Seems like it's a delicate balancing act from sanding with enough pressure to be effective (which doesn't seem like much) and putting too much pressure and burning thorugh the previous coat completely. I always end up with dust/white residue from lightly sanding and assume this is some of the Poly coming off...
Sanding a self-leveled finish coat will produce scratches, that's what sandpaper does. The white dust IS poly coming back off.
I apply several coats before the first sanding, I just de-nib with brown paper to get off floaters. By my first sanding I have enough base not to burn through to wood. As ysu65 says, go with a very light touch.
How you wrap your sanding block matters. I use a stiff cork block and wrap the paper in the direction of travel. My block has a slight curve to the leading and trailing edges so the paper has no hard corner folds. Corner folds can protrude down and leave heavy scratches.
If you are wrapping paper around a square block that could be the cause of an uneven scratch pattern even when using a light touch.
Thanks very much for the feedback. So you apply several coats of Poly (letting each one dry) and don't sand in between each coat, and then at some point you do sand it before applying your final coat?
I was using a 3M sanding block, however the paper was not tight on the ends, so I wonder if there was some kind of chattering while sanding that may have caused these extra scratches. They are not everywhere, only some areas I can see them. I have found a 3M rectangular sanding sponge that I am thinking of wrapping the sandpaper over to use.
When you say you wrap the paper in the direction of travel, you do this so the leading edge of the paper isn't contacting the surface? Is it ok to sand in between coats with 320, then 400 before applying the next coat? Sorry for all the questions, just really would like to get it looking nice.
As someone else said there are many options on how to apply finish. Here Is what I like for poly. I'll thin the poly quite a bit so it self levels better and dries faster. I've been using a blend of 3 parts mineral spirits, 2 parts poly, 1 part tung oil, but I think 1-1 mineral spirits to poly would be close. I make sure I've got a good surface before I start then wipe on the finish, and then sand with 600 wet dry sand paper while it is fresh, then wipe off any excess several minutes.
The first coat has a little bit of wood dust but it is kinda like french polish in that the dust finish mix helps fill the pores. I do the same procedure sanding in the wet finish for each coat. The final coat possibly switching to 800 if I feel like it. 10 coats or so. The light coats mean I never build enough I have to much to smooth out between coats and it gets just enough tooth to stick and the nubs get knocked down.
I don't usually use poly, preferring wipe on varnish like Waterlox. I have discovered that there is a big difference in the resulting finish quality after sanding if I use wet-or-dry paper lubricated with the solvent of the finish (for Waterlox it is mineral spirits.) Dry sanding leaves scratches that can occasionally not fill well with the next coat, especially on highly figured wood; wet sanding leaves a smoother, visible scratch free surface. You can start to hear the difference when the lubricant starts to be insufficient; then it's time to add more lubricant. You can reduce the scratches even further by rubbing with 0000 grade steel wool after the sanding, which will leave a uniform haze (i.e., very small scratches not actually visible as individual scratches). The steel wool doesn't do as good a job at leveling the surface as sandpaper and a block, so should be used after sanding, not alone. Some steel wool has a lubricant/oil on it; it's safer to wipe the surface down with your solvent before applying the next coat. This also removes all the dust residue.
Happy finishing!
Instead of using 0000 steel wool as you suggest, I have better luck with 3M Scotchbright. The dark red is equivalent to 300 grit paper and the dark gray is equivalent to 600 grit paper. (there is also a white variety that is about 1000-1200 grit I believe) For the most part I use it to "scuff" the surface between coats of finish to promote adhesion. Compared to same-grit sand paper the Scotchbright leaves a much more consistent surface devoid of the heavy scratches that paper often leaves. Scotchbright also tends to more fully "scuff" the surface then paper. I usually use it in conjunction with a foam pad of some type but occasionally (especially in tight areas) just use my fingers. Scotchbright can be used wet or dry and will last much longer when used wet with the solvent of choice. Unfortunately, if attempting to remove heavy flaws in the finish (runs, drips, etc), Scotchbright is not the best choice (just like steel wool) and you will have to resort to paper abrasives before using Scotchbright. For me, the best source of supply is a local auto body/paint supply store.
If your getting spots worthy of sanding to level it sounds like you're putting it on too thick. Thin coats. Spread it on wipe it off lightly and let it level. (I use minwax wipe on poly) Super light sanding (320 in one pass) just to knock the nibs off and give it an almost undiscernible scuff and put on the next coat. It doesn't have to be all cloudy to hold the next coat. I usually finish it off with 0000 and paint thinner as a lubricant light/medium pressure gives it an even satin baby butt smooth sheen. Thin coat of wax and call it done. That's my process anyway.
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