Final Wet Sand on Polyurethane Finish - Woodworking Talk - Woodworkers Forum
 
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post #1 of 7 Old 08-21-2013, 10:02 PM Thread Starter
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Final Wet Sand on Polyurethane Finish

Please help:

I am a beginner woodworker. I'm re-finishing a table, and am working the final sanding of my polyurethane layer. I've never finished anything before, so I of course gave myself the Google U. educational course. I have 5 layers of Minwax Wipe-on oil based poly (satin finish) on the table, having sanded with 220 lightly (in the direction of the grain) between each application.

After the last coat, I let the table dry for 2 days, and went out this evening to try the final sanding/finish. I thought I'd try a wet sanding technique. Googled that too and followed what I saw/read about how to do it. I started with a 320 waterproof sand pad. Lubricated with olive oil (I didn't have mineral oil). At first I thought it looked real nice, but when I wiped all the oil off, I noticed the scuff marks were very visible when not looking at the table straight on (see pic below)

Unlike every other sanding, I used a circular motion this time. My question is, "how do I fix this?" Things I'm wondering:
  • Is this normal for 320 grit?
  • Will the scuff marks dissapear as I transition to a higher grit paper?
  • Should I be using a circular motion?
  • Mostly… did I just ruin my finish?
Any help/advice from you experienced artisans would be greatly appreciated. Thanks!

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post #2 of 7 Old 08-21-2013, 11:47 PM
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I don't think it was a good idea to use olive oil in the sanding process. I would wipe the table down with naphtha if you have it or mineral spirits would be my second choice. Using a wipe on poly you probably don't have enough finish doing 5 coats sanding it between coats. With a wipe on poly 3 coats usually equal 1 coat of brush on poly so since you sanded between coats you probably are at about 1 coat. About the scratches I think you need at least two more coats from where you are so I would go ahead and apply a coat and see what it does for the scratches. If they are still showing wet sand it again with 400 grit going with the grain and put the seventh coat on.

What about the knot hole? Most of us when we build a table will fill a knot hole. You could mask around the knot and fill it with walnut wood putty thin layers at a time until you fill the hole and sand it flat with the table top. Then put a couple of coats of the wipe on on just the spot before you continue.

Last edited by Steve Neul; 08-21-2013 at 11:50 PM.
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post #3 of 7 Old 08-22-2013, 12:33 AM Thread Starter
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Thanks Steve,

I have some mineral spirits, so I'll use that to clean off the table. Probably need to wait until tomorrow at this point to do any more coats. On and off, I've been regretting using the wipe-on stuff. Would there be any issue switching at this point to a brush on. I know the dry time between coats is longer, but I like the idea of it going on thicker. I'll also pick up some mineral oil tomorrow and use that in the wet sanding from now on.

As far as the knot hole... the table is for my wife. It's actually just a re-finish of an old table that my wife wants to use for a sewing table. I made my own wood filler from the initial saw dust to fill in the ends of the top where the laminations had separated (the table actually sat unfinished outside for an entire Oregon spring with lots of rain before I started this project). She liked the knots however... they do look kind of nice in a rustic way along with some small checking on another part of the top.
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post #4 of 7 Old 08-22-2013, 09:03 AM
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Quote:
Originally Posted by wilbrijo06 View Post
Thanks Steve,

I have some mineral spirits, so I'll use that to clean off the table. Probably need to wait until tomorrow at this point to do any more coats. On and off, I've been regretting using the wipe-on stuff. Would there be any issue switching at this point to a brush on. I know the dry time between coats is longer, but I like the idea of it going on thicker. I'll also pick up some mineral oil tomorrow and use that in the wet sanding from now on.

As far as the knot hole... the table is for my wife. It's actually just a re-finish of an old table that my wife wants to use for a sewing table. I made my own wood filler from the initial saw dust to fill in the ends of the top where the laminations had separated (the table actually sat unfinished outside for an entire Oregon spring with lots of rain before I started this project). She liked the knots however... they do look kind of nice in a rustic way along with some small checking on another part of the top.
No, you don't want to use mineral oil either. Any foreign substance can interfere with the adhesion of the finish or cause some adverse chemical reaction. Use only water for wet sanding between coats. I think it's a better lubricant anyway.

You could change to the brushing poly but it can sometimes be a pain as well. If you don't have a soft enough brush or it's too hot or you don't work it fast enough then you are left with a bunch of brush marks to contend with. The wiping poly is the easier product to get good results short of spraying the finish. If you are going to brush it do it sometime when the temperature is under 90. Find a paint brush that is super soft to apply it. Keep in mind that as soon as the poly hits the table it is starting to dry so don't stand there and brush it and brush it because that introduces air into the finish making it set up faster and makes brush marks. You have to lay it on with as few strokes as you can and move on. The finish will flow out as it sits. If you miss a spot don't go back and fix it. Wait until it dries and get it on the next coat. If you just let it dry the spot would be minor and perhaps you could buff out but if you go back it will make a textured spot from the brush that will have to be sanded out.
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post #5 of 7 Old 08-24-2013, 11:10 PM
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I have never heard of any one using olive oil to "wet sand". I have only used water with a 400. Resand, wet, with a 1200. I'm a rookie. It's been my experience to only wet sand with a high grain paper. I maybe talking out my blow hose..
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post #6 of 7 Old 08-24-2013, 11:48 PM
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Quote:
Originally Posted by HandymanMatt View Post
I have never heard of any one using olive oil to "wet sand". I have only used water with a 400. Resand, wet, with a 1200. I'm a rookie. It's been my experience to only wet sand with a high grain paper. I maybe talking out my blow hose..
I've never heard of it either but he may have read it somewhere where someone was sanding as part of hand rubbing where there was no intention to putting a finish over. If it wasn't between the coats sanding I wouldn't think it would hurt anything.
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post #7 of 7 Old 08-25-2013, 12:23 AM
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You may have sanded deep into the finish since wipe on poly is like 3 coats of wiping to one coat of brush on.
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