Wipe-on polyurethane is a hard, easy-to-apply finish with a lustre that ranges from a satin sheen to a bright gloss. Until recently, it was the secret weapon of only those who knew the recipe. Now it’s on the shelves of every paint store, hardware or home centre under brand names such as Minwax and Varathane. While it’s just about the most idiot-proof finish on the market, there are still a few tricks to using it successfully. If someone had been able to give me an idea how this stuff works from start to finish before I began using it, I would have saved myself a lot of time learning by my mistakes.
Test Pieces...
The first thing I recommend is to use lots of test pieces. Perform the finishing schedule as you expect to do it for the finished piece (More on possible finishing schedules below). If you're in no hurry you can do a couple of test pieces through the entire finishing schedule first to see how it works out. I suggest doing this with two pieces because there will always be times when you ask yourself questions like, What happens if I use 220 grit, then 320 grit, or if I just jump straight to 320 grit? Or what happens if I get a little heavier with the next coat (or two or...)?
Once you gain a little more experience, or are in a bit of a rush, you can just use one or two test pieces and do each step one step ahead of the real piece. That is, do the first coat on the test piece(s) but don't do the first coat on the real piece until you're ready to do the second coat on the test piece. Seems a bit more work but this simple extra step can save you a lot of work in the end! (Don't ask how I know...)
It's probably obvious but I'll say it anyway... Always use the same wood for your test piece(s) as the real piece. If there are suitable pieces from the same board(s), that's ideal.
So here goes...
Sand the wood through the grits you plan to use. (Up to150 or 180 is fine enough unless you have a specific reason to go finer.) Then apply the number of coats you think you will need. Test pieces are especially important if you have no idea how many coats will be required to achieve the finish you are seeking.
Be aware of the recommended “recoat” time on the can. It’s typically two to four hours, but will vary according to the temperature and humidity in your shop. If you apply a following coat in approximately the recommended recoat time you do not need to sand before every new application unless you have a specific purpose, e.g. to remove any dust nibs or other roughness. In any event, wait until you can slide a finger over it without it sticking. If it’s sticky to the touch anywhere, don’t recoat yet. If you plan to sand before the next coat, be sure to wait long enough that sanding produces a fine white powder.
If you wait significantly more than the recommended recoat time, e.g. ten to twelve hours or longer, then you need to sand before the next application. Each layer of polyurethanes (oil- or water-based) bonds to the previous layer mechanically, sort of like epoxy, so there needs to be some “tack” or “tooth” for the mechanical bond. If you are only sanding because you waited longer than the recoat time, - e.g. you had to get some sleep! - you only need to scuff sand. That is, use a fairly fine grit – say, 220 or 320 – and sand very lightly. Remember, each coat of wipe-on poly is very thin. You don’t want to get aggressive and sand through the previous coat for no reason.
Consider using Gloss for all but the last coat or two, even if your goal is a satin sheen. “Satin” and “Semi-Gloss” urethanes have particles in them to dull the finish. If you use them for the entire finish you can end up with a finish that might not have as much depth as you want.
A characteristic of wipe-on poly that makes it so user-friendly is that it is very thin. This is what makes it easy to apply and allows it to dry quickly. But if you are accustomed to using paint and regular polyurethane finishes that are done in a coat or two, you need to be aware that wipe-on poly might take two or three coats before you even start to see a film building on the wood. That’s just the way it works. Don’t panic and start sloshing the stuff on. It dries quickly and the application of multiple thin coats is the way the finish is intended to be developed. It enables you to sneak up on just the quality of finish you want without fear of runs, or of overshooting and making your piece look like you dunked it in plastic.
I use cast off cotton/polyester sheets or t-shirts for wiping pads. I generally take a piece about 6” x 6” and fold it so that no raw edges are left out to come unravelled and leave threads in the finish. To make it easier to hold, I picked up a 7” curved hemostat to hold the folded pad. That way I can use it almost like a paintbrush and I don’t need to wear gloves.
A hemostat looks like a pair of scissors but it has jaws like mini pliers and the handles lock closed. You can get them in drug stores and even surplus stores like Princess Auto in Canada.
SAFETY HAZARD: I toss the pad after every session (two or three applications). Do not throw the damp pads in the garbage or you risk starting a fire from spontaneous combustion. Open the pads up and let them sit somewhere to dry completely before discarding (two or three days to be safe). You can toss them on a concrete floor or into a bucket of water. You should take this precaution with rags that have been used on any finish with oil in it.
Don’t be afraid to brush wipe-on poly when that seems to make the most sense. Getting the first sealing coat on a large flat surface is an example. Just don’t overdo it and do watch for runs. I use artists’ brushes to get into corners and tight, odd-shaped spaces.
Example Finishing Schedules...
Here are a couple of examples of finishing schedules that I use. They hit two extremes so yours will probably land somewhere in the middle.
The first example is my daughters wedding card box.
She wanted it to have a natural look and feel. "None of that shiny plastic look, Dad."
To start, I sanded to180 grit. I did two coats of wipe-on poly spaced about 2 hours apart in the first session and then left it overnight.
The next morning I sanded very lightly with 220 grit - just enough to take any roughness off but not enough to take the wood grain feel away from it – and applied one more coat of wipe-on. I let it dry completely (24 hours) and gave it one last very light touch with 320. Just enough to take the dust nibs off. It looks natural and feels like wood, but it has a soft sheen that will last, and enough protection for something that will be looked at more than it will get used. She loves it.
The hardest part of that finishing schedule was combating the urge to do “just one more” coat. This is where the test pieces are so important. I knew that just one more coat would produce an obvious film build-up … just the look my daughter did not want.
The second example is the one I use for most of my band saw boxes.
My band saw boxes get an average of 8 to 10 coats, sometimes more. I want them to have the finished surface completely smooth and a deep but not glaring finish.
The first session will get two or three coats and then dry overnight. I will normally sand after the second coat to knock off any grain and get as smooth a surface as possible – usually with 220 grit. At this point I’m using the poly more like a sanding sealer. It will sit overnight after the second or third coat, depending upon how close it’s getting to bedtime!
Because the recommended recoat time has passed, in this session they get sanded in preparation for the next coat. It will be either 400 if they are already reasonably smooth, or 320 then 400 if they are a bit rough. Then I'll put two or three more coats on. In this session, if I get started early enough in the day, and if the film is starting to build relatively evenly after the second coat of the day, I'll let it dry enough to sand, usually around four hours. I’ll sand with 400 or 500 and put a third coat of the day on. If it's not yet building evenly, I'll skip the sanding before the third coat, and then let it sit overnight.
The next morning, it needs scuff sanding before the first coat of the day. I sand first with 500. If necessary, e.g. if there are some fairly open pores in the grain that need to be levelled out, I might sand an entire coat off. (Remember I said I want this thing smooth when I'm done.) With this particular finishing schedule I’m using the poly as grain filler until I get the surface perfectly smooth. With difficult woods, if the grain is not filling as rapidly as I expected, I might even use 400 grit to take the finish back almost to the surface wood.
If the surface is becoming level and smooth, it will be a similar day to the previous. I’ll do two or three more coats. They will go quickly because at this point the film has built evenly, and all I'm doing is any final levelling that's needed, while watching for the depth I want. (This is where the test pieces prove invaluable. Do a test piece right along with the project piece. If you do each step on a test piece and check the results before doing it to the project piece, you can see what will happen before you risk screwing up the project piece.) I might decide to stop after two if it's looking close, or it might get the third. Again it sits overnight.
This should be the final session. Sand with 1000 or 2000, depending on how smooth it already is. (Remember, I said this was an extreme example!) 1000 if I think it needs levelling out a bit. 2000 if it's ready for the last coat. If it's ready, it gets the last coat.
Shiny or not...?
Sheen is a matter of personal preference. I don't like gloss for many woods (although some, like the cherry box above, demand a nice glossy finish). So I use gloss for everything up to the last coat, then switch to satin. Sometimes I'll do two coats of satin if required to kill the glare.
Instead of applying a coat or two of satin, you can do the same thing by “rubbing out” the final coat. Wait at least a day or two for the last coat to cure well. Give it a very light sanding with 2000 or 1000 sandpaper (I use Abralon sanding pads for the really fine grits) or 0000 steel wool on the last Gloss coat instead of switching to satin. Remember that last coat is very thin so don’t get aggressive and take it off. You just want to soften it to a lustrous sheen.
On the other hand, if you make to sell, be careful that your own preferences don’t over-ride those of your customers. Lots of people really love gloss, so finishing some of your pieces to a very high sheen could result in sales you probably wouldn’t have made if you offered only satin.
Sanding...
I do two kinds of sanding in finishing sessions. If I'm simply scuff sanding to take off the dust nibs and scuff it for the next coat, the touch is very light. Very light! You want to take any roughness off, so it’s smooth to the touch, and produce just a bit of tooth for the next coat to grab onto, but you do not want to actually sand any finish off.
If I'm trying to level it, then I'll get more vigorous. I don't usually want to sand back to the raw wood, but I'm not concerned about going back into previous coats. The one thing you really need to watch out for here is that if you sand back a coat, you pretty much need to take that entire coat off the whole piece. If you are doing a really nice job of finishing, if you don't take the coat off the whole piece, or at least in the visible areas, you may end up with the edges between coats showing up. If I didn't do a good job of filling the pores of the wood in the early coats, it isn't unusual for me to sand it nearly back to bare wood and cycle through two coats and sand them off, two coats and sand them off until I'm satisfied with the surface before continuing on and letting the film build up.
There is lots of room to experiment with wipe-on poly, and it is forgiving. However, I hope this helps you get through the learning curve a little quicker than I did.
Special thanks to Brian Knodel in Dawson Creek, BC. We went through this learning process together, and shared a great deal of both the joy and pain of finding out what does and does not work.
