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Sanding between coats of Minwax Polycrylic

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Forum topic by gauntlet21 posted 07-03-2018 05:53 PM 8335 views 0 times favorited 9 replies Add to Favorites Watch
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gauntlet21

69 posts in 665 days


07-03-2018 05:53 PM

Topic tags/keywords: finish clear brush marks brush strokes maple stain

I’m certainly new to woodworking and as I complete what I would consider my second project (my first project was a simple rolling stand for my lunchbox planer made of pine 2×4’s and a plywood tabletop that I colored with Teak Oil and Minwax Polycrylic) I am hitting what I think is a snag. I’m building a cubbie bookshelf made out of Maple Veneered Plywood (3/4”). I have learned a lot already through this process but am not certain as to how I can remedy something in particular. There are two parts to my question. First, assuming you are going to apply 3 coats of a clear coat finish, when you sand inbetween each coat, should you see the sporadic scuff marks that “look” like you’ve removed the clear coat? I am now only using a Preppin’ Weapon Sanding Block with 320 grit sandpaper after my second coat of clear finish and even with as light of sanding as possible, I still discover spots (under the right reflective lighting) where it looks as if I have removed the clear coat. My second question is, even with as much attention to brush strokes as possible, when I get to an edge or corner and apply the clear coat, I am not able to apply it with a long brush stroke. Instead it is applied with short strokes or dabs of the paint brush and then I have to clean up a vertical edge that begins dripping. This has led to visibly, non-continuous areas in the clear coat (again, under the correct reflective light). I realize that if the clear coat isn’t right, I can do a 4th coat or 5th if necessary but it’s really not the most enjoyable part of woodworking. I have 8 boards that are 43”x15” and both sides need the finish so it takes a while to complete a single coat. I’m just not thrilled with how the stain and clear coat have turned out. Some of it is my fault for not knowing that Maple receives stain like Scotchguard retains spilled nachos and after I applied my first clear coat, I used my “finishing sander” which removed essentially all of the first coat along with portions of the stain. I’m now saying that I am going for a rustic look (like designer jeans nowadays). Anyway, if anyone has recommendations to eliminate some of these problems, I’d love to hear them. Perhaps I need to tape the edges while applying the clear coat to the faces of the boards so I don’t have to worry about leaving drip marks on the edges OR having an uneven flow in the clear coat.

I appreciate any guidance.

Thanks,

Dan


9 replies so far

View bondogaposis's profile

bondogaposis

5496 posts in 2806 days


#1 posted 07-03-2018 09:16 PM

It sounds like you are sanding right through the finish. There is no point in multiple coats if you are going to sand it off. When sanding between coats don’t use the sanding block and use a very light touch. I use 400 grit foam backed sandpaper for that task. Another option is the non woven scotch brite pads. Regular sandpaper will work too, but just use it in your hand so you can feel what is happening and don’t bear down on it. To answer your second question, the best solution to apply finish to the insides of a project prior to assembly. Inside corners are a bugger as you are finding out. Of course it is too late for that now. I recommend that you mask off the vertical surfaces and apply finish to the horizontal surfaces. When that cures rotate it 90° and repeat.

-- Bondo Gaposis

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woodbutcherbynight

5967 posts in 2864 days


#2 posted 07-04-2018 03:09 AM

Sound advice. Scotchbrite pads are your friend!!

-- Live to tell the stories, they sound better that way.

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becikeja

1005 posts in 3268 days


#3 posted 07-04-2018 01:18 PM

I struggled for a long time with applying the top coat. I started out by sanding as the directions said and had the same results you’re seeing. Then I decided not to sand between coats at all, and found that I could never get a smooth finish.
Then one day I was at a woodworking show and discussing my dilemma with one of the speakers. I remember him looking at me like I was crazy and had no business in woodworking. He then arrogantly told me that the idea of sanding between coats was simply to knock the grain back down and break the seal for the next coat. The first coat filled the pores, the second coat would fill gaps caused by the first sanding and the 3rd coat and/or 4th coat actually provided the finish. Hmmm I left puzzled and then the light turned on for me. I was looking for a finish from the first coat and should have just been trying to establish a base. Now I sand my projects to 220. Then apply a light first coat rubbing it on and into the wood with a cloth, not a brush. After the first coat a very light hand sanding with 400. I simply try to get a little dust stirred up and cloudy look to the finish and sand till its smooth to the touch which doesn’t take much. I then blow the dust off with an air gun, and repeat till the shine I want is there. Most of the time 3 coats does the trick but sometimes I need 4 depending on the wood. I always use General Finishes Arm-R-Seal if I’m using polyurethane. I’ve never been able to get a good finish with Minwax. Just saying.

-- Don't outsmart your common sense

View theart's profile

theart

112 posts in 1009 days


#4 posted 07-04-2018 03:40 PM

Think about it this way. A dry coat of poly is around 0.003” thick, and the grains on 220 sandpaper are 0.0045”. It really doesn’t take much to sand right through, particularly on outside corners. 400 or Scotchbrite between coats, and just enough to rough it up so the next coat sticks.

Also, I’ve found that the only thing that gets me a good top coat with Polycrylic is a 6” trim roller.

View Rich's profile

Rich

4701 posts in 1044 days


#5 posted 07-04-2018 04:05 PM

I think it’s usually best to follow the manufacturers directions. They say to sand with 220 between coats, letting each dry at least 2 hours. They also say 3 coats are recommended.

However, the 220 grit suggestion sounds fishy to me. It seems too coarse. Also, Minwax has one of the poorer reputations for good advice. That’s just an impression I get from reading, I have nothing concrete to back it up.

If it were me, I’d use 400 grit without a sanding block and just go over the surface lightly to knock off any nibs. I’ve gotten excellent results with waterborne poly as far as the quality of the surface goes doing that. What I don’t like about them is that the non-gloss sheens seem to leave a bit of a haze after a couple of coats. Behlen has been the worst leaving an unacceptable blueish haze in the finish. General Finishes High Performance is better, but for my money, I prefer to stick to oil based products like Arm-R-Seal when I need a durable finish for a surface. All other times, I use lacquer for its ease of application and flawless finish.

-- My grandfather always said that when one door closes, another one opens. He was a wonderful man, but a lousy cabinet maker

View William Shelley's profile

William Shelley

609 posts in 1924 days


#6 posted 07-04-2018 08:08 PM

Grit aside, my experience with waterborne polyurethanes is that they take a while to harden up after drying. If you sand too soon, the finish is gummy and will clog up finer grit sandpaper almost immediately.

As I understand it, the urethane resins in the waterborne products still need to cure via oxidative reaction just like the oil-based counterpart. The difference is that the resins are dissolved in glycol ether, which is then mixed with water to create the product in the can. The water evaporates quickly but the cure still takes time. I also don’t believe that you need to sand between coats of waterborne poly, except to knock down fuzzy grain. So between coats 2 and 3, you could probably get away with no sanding.

-- Woodworking from an engineer's perspective

View Richard's profile

Richard

11291 posts in 3487 days


#7 posted 07-04-2018 08:12 PM



Think about it this way. A dry coat of poly is around 0.003” thick, and the grains on 220 sandpaper are 0.0045”. It really doesn t take much to sand right through, particularly on outside corners. 400 or Scotchbrite between coats, and just enough to rough it up so the next coat sticks.

Also, I ve found that the only thing that gets me a good top coat with Polycrylic is a 6” trim roller.

- theart

I’d go with this!

Rick

-- Richard (Ontario, CANADA)

View fuigb's profile

fuigb

559 posts in 3412 days


#8 posted 07-05-2018 12:05 AM

I sand between coats of poly principles to knock off dust that settles on the coat just dried. I see how Scotch Brite pads may do the trick but I just use a really beat up scrap of high-grit paper, usually 220. By beat up I mean the stuff that normally you’d have thrown away because most of the grit is gone. A light sanding, vacuum of the dust, clean with mineral spirits, wait a few minutes for things to dry, then on to the next coat. Don’t sand through the poly, and after the last coat polish with 0000 steel wool.

-- - Crud. Go tell your mother that I need a Band-aid.

View Robert's profile

Robert

3485 posts in 1935 days


#9 posted 07-05-2018 02:41 PM

Dan,

I like the Rhynosoft foam sanding pads.

I think 220 is too coarse stay with 320.

Do it all by hand, of course.

-- Everything is a prototype thats why its one of a kind!!

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