I promised a novice floor guy in New Jersey to write this article. He's been having quite a problem, (and I'm sure customer complaints) because he has been leaving bubbles in the last coat of polyurethane. He would like a quick fix, but I'm afraid both you and he may be a bit surprised by the length and detail in this article. I sure that you will be satisfied that I covered all the problems associated with this fine and durable finish.
Polyurethane's official name is actually Oil Modified Polyurethane, or OMP or OMU. That's because it is mainly made up of oil (safflower or Soya oil), some metallic dryers (cobalt and manganese) and lastly some polyurethane resin to give it hardness once it dries. It's also referred to as urethane, or even trade names like Varathane. You can be sure you are buying an OMU when the directions on the can suggest clean up with mineral spirits. Water based finishes are also called urethanes in some cases, but I will not talk about these finishes here. They have a completely different application method. It may become confusing when water based finishes call themselves urethane. But the water base finishes always suggest a soap and water clean up. Whereas oil based polyurethane's require mineral spirits, naphtha or paint thinner for clean up.
But lets start from the beginning. If you are sanding a floor for a clear finish (some call it a natural finish) you must sand the floor at least 3 times. Start out with 36-grit paper, then 60 then 100 grit. If the floor is to be stained you must skip less grits and go with 36, 60, 80, then 100 or 120. The edges must be sanded thrice at least, with the final edging done with a 1/2-sheet vibrator sander with 80-100-grit paper. And for stained floors you must screen the bare wood with 100 grit screens just before staining. I'm not going to give you a sanding lesson in this article that will be for another time. Suffice it to say if your floor is not properly sanded clean and fine, your finish will always be rough. Oh, that's why I always use 36-grit sandpaper on old and new floors. On old floor I need to remove ALL the old finish with the first rough sanding, no exceptions. And on new floors I want to level the boards quickly and easily.
So now that you have a nicely sanded floor, lets start and say you are doing a clear finish with 3 coats of polyurethane, what is the best way to proceed. First, sweep the floor if it is really full of dust, as this may be too much for your vacuum. Vacuum with really good industrial machine. It would be worth it to rent one for the whole job. There will be large particles of sandpaper grit on the floor, and your household vacuum wont do the job. Be sure to use the wand of the vacuum to get into all the corners. Keep in mind, its best to go over the floor twice, this will ensure the floor is dust free. By the way, you need to finish your sanding the day you do the finish. Never leave a fine sanded floor overnight, any dampness in the air may raise the grain, especially in the summer.
I like to apply my first coat of polyurethane with a 12" drywall trowel. I usually thin the finish I use (Fabulon Brand heavy-duty poly satin) about 3-5% when I am troweling it. This particular brand has a bit too much body for troweling. In my case want to slow the drying down on this first coat, so I add odorless mineral spirits. I want to prevent trowel marks. I pour a puddle of this slightly thinned finish on the floor, and work it back and forth (just like glue troweling).
Another method is too simply brush a coat diluted about 10-15% with the thinner that is supplied by the manufacturer. This thinner will speed up the dry time only just a little. But applying a thin coat is the important thing here.
Only VM Naphtha will speed up dry times, and odorless mineral spirits will slow it down. So, in the hot humid summer time you may want to use naphtha, or else the finish will take too long to dry. But in winter, in a warm, overly dry house you might want to use odorless mineral spirits, to slow the drying of this first coat, so that it flows better. I avoid all of this by using the trowel, and apply this first coat of finish THINLY instead of thinned. Its a skill a pro should learn.
Some manufactures refuse to supply any thinner with their finish. They site the VOC regulations and state in their directions that the finish is NOT to be thinned. Boy is this dumb. If you insist on applying the first coat full strength you will in most cases have a bubbled and pitted and very slow drying finish, but you will be following directions. Good for you. In this case try to get the best paint thinner you can buy, when you wish not to speed up or slow down the dry time like I mentioned earlier. You want in any case, to apply a thin coat, no more that 700-800 square feet per gallon.
AND IN EVERY CASE, NEVER USE A LAMBS WOOL APPLICATOR. Brush sparingly in any direction to wet the wood, them comb once with the grain. Dont play around with the finish it will only cause more bubbling. And that is why I always trowel on my first coat, I dont get any bubbles with this method. A pro should learn to trowel, it saves finish and time.
The reason for this thin coat is simple. The first coat of poly is only meant to fill up the woods pores, and bring the finish to the surface of the wood. Any more than this and most finishes will get rough and pitted. So if you apply a heavy coat of finish to bare wood, it will dry so rough that in most cases you will have to sand almost down to the wood surface anyway. And if there are contaminants in the wood, (waxes, silicones and oils) this heavy first coat will take a long, long time to dry. The drywall towel applies a thin measured coating of poly, just enough to top up the pores of the wood, and it most always dried overnight in a warm 70 F floor. Notice how I say a 70 F floor, because if you have a crawl space under the floor, your floor surface may be too cold for any finish to dry properly. The wood itself needs to be 70 F.
I use the same finish for all three coats. This makes touch-ups a lot easier in the future. I never use a special oil based sealer, this is just a thinned finish and as you see you can make your own. And never use a lacquer sealer. Just read about this terrible and flammable stuff in the Floored News section.
And thats the key to a successful finish, each coat must dry thoroughly before you apply additional coats. How do you tell if a finish is dry? Well and experienced floor mechanic will smell the excess solvent in the floor, and sometimes just feel the finish; it may have just a touch of tackiness to it. But the acid test is to get the floor buffer on it with 100 -120 grit screen. Start in the far corner of the room. If the finish gums up you wont make too much of a mess. Run the buffer on the floor for about a 1/2-minute gently moving it back and forth. Stop and check the screen. If it shows any signs of gumming up, quit and wait another 24 hours. Most instructions on finish can labels are way too optimistic, and dont allow for the variables that will occur in floor finishing. Floors are large areas, so at the very least that can double the dry time (as say a tabletop) as there is a lot of solvent to be released. And there are always contaminants in old floor and sometimes new floors. Some tropical wood cannot be coated with poly, and even eastern white pine with its numerous pitch pockets can take days between coats, to set up. The same hold true if you are simply sanding by hand between coats, if the paper gums up, and the finish doesn't powder, wait at least 24 hours. Too many problems in the next coats are caused by failing to let this coat dry out. Failure to head this warning will make at best a weakened finish or at worst top coats that wont dry.
And this goes double for those you who want to stain the floor before you apply three (yes 3) coats of poly. All pigment stains contain a binder that allows the pigments particles to stick to the pores of the wood. This is a form of varnish, in the slower drying so called oil-based stains. And in all cases this stain needs 24-72 hours to dry. If you coat over a stain too early it will definitely interfere with subsequent coats, and you will have no end of problems. Its typical that the poly wont dry or stick to an undried wood stain. Know your stain, and its compatibility with the finish you are using. Do a test in the corner of the room. Let the finish cure (15-30 days) then do an adhesion test. Cross hatch the test spot to the bare wood with a razor apply duct tape and rip it off. If it peels use another stain or finish. If you are using the same brand of stain and finish, most times you will be OK. But with differing brands, you will take your chances until you do the above test.
I solve this by applying a furniture quality "fast dry" wiping stain. I use a local manufacturers proven product. It has the advantage of drying in the lighter colors in about 4 hours, and the darker colors always dry overnight. The quick drying solvents in this type of stain make it a bit trickier to use. It contains fewer binders, so you have to be really careful the next day when you walk on this stain. The poly, if brushed too vigorously, will wipe away this type of stain. But I am a seasoned professional, and can handle these limitations. I can in most cases, stain and coat the same day. I always apply the stain with lint free rags, and vacuum after the stain is dry, and brush on the poly full strength. The reason for this full strength coat is simple; I have to build my finish over the stain quickly, so that when I scuff sand (or screen) between coats, I wont rub out the stain. The binder in the stain has helped fill the wood pores a bit, and the extra fine sanding has made the first coat of poly on the stain go on quite smoothly. So in this case never trowel a finish over a wood stain, and never add thinner to it either. Im going to do a separate article on sanding and staining floors, once I get enough requests.
And now, once you are quite certain that the fist coat of finish is quite dry, you can proceed in the screening. Use your buffing machine with the handle low to the floor, you dont want to gouge too much finish off the wood, after all this is only a thin coat. If you have stained the floor, and are buffing the first coat, you must take even more care. Run the buffer in a corner first, and I suggest you dull the screen by sanding the screen itself. See if you are removing too much finish and are thereby removing the stain to the bare wood. This is a delicate operation. On the one hand you want to scuff the finish. Poly just will not stick to itself otherwise. But you dont want to sand though the finish into the delicate stain. You may have to hand screen the floor instead. Thats why I always apply a smooth but heavy (500 sq. ft. per gal.) coating over my dried stains. But I still take care, and thats why I charge almost double to sand and stain a floor. Its a risky job.
Run the buffer in both directions never stopping in any spot, just a nice easy back and forth motion. Go with the run of the wood, and then repeat again against the run of the floorboards. This assures you that all low spots on the floor will have the finish well scratched. Next, get on your hand and knees and scuff sand the floor in all the areas that the buffer couldnt reach, and make sure to go all along the edges. This gives you a chance to see if all the pits, lumps and blobs are sanded down smooth and flat with the surrounding finish. You may have to gently scrape out some blobs, and really sand those pits until they are not visible. The more fussy you are now the better the next coats will come out. This is precisely why you need a really dry finish; else all this scuff sanding will just make a gummy mess. Be patient.
One you have a 100% scuff sanded finish, and it feels as smooth as a babys behind, you can start cleaning up. Vacuum twice again, with your industrial vacuum. Get into all the corners of course. Now you can tack rag the floor. I use white lint free rags (yes you can get lint free cotton rags at most big box stores, expensive but worth it). Choose the most lint free in rag in the box, and wet it well with paint thinner. In this case paint thinner is just right, it dries at just the right rate for cleaning the floor. Fold the rag in a long narrow bundle and using both hands on your knees wash the floor in a back and forth sweeping motion as you crawl backwards. I wear my ProKnee kneepads so even after 23 years on the floor; I can still do this with comfort. And hey, Im not here to tell you the easiest way to prep a floor, just the best way.
I started tacking between coats of poly 21 years ago, and doing this on your knees is the best way to clear that fine powdery dust that the vacuum cannot suck up. Let the solvent dry completely. Dont coat over any solvent, this will interfere with the poly and cause bubbles. You want the second coat to be almost perfect. When the solvent is dry and before you coat the floor check for dust and debris one more time, with your hand. If it is still dirty, do one more tack ragging.
Now we come to the fun part, brushing on the finish. Yes I said BRUSHING. I quit using lambs wool applicators about 10 years ago, when I tried to clean one of these dirt collectors. After only 6 months of use (and cleaning between uses) I soaked it in solvent, and try as I might could not get all the dried caked finish globs and bits out of the interior of the lambs wool pad. Instead I purchased an 8" Embee natural bristle brush, and have always been able to clean this free of all contaminants. I now also have a 12" short bristle brush that I use on small jobs by hand. The longer nap Embee brush can be taped onto a pole and is almost as efficient as the old lambs wool applicator, without making the bubbles or leaving behind the junk. If you wish not to invest in $100 brushes at this time, I would recommend the 4" Purdy brand bushes, with the unvarnished wood handle. You may have to spend 30 dollars on a brush like this but you will never wear it out.
Now heres your floor-brushing lesson. Filter the finish into these large aluminum-roasting pans. I buy my floor finish in 5-gallon containers, so I just have to put a few layers of cheesecloth over the spout held on with a rubber band. You do need to filter these reactive type varnishes because they are always in some state of curing some dried film in the can. The older the can the more junk is in it. I pour the finish slowly into the shallow roasting pan, so I dont create bubbles in the finish. The shallow pan will release the bubbles faster in any case than a deep bucket. If you are using a satin finish (which I always do) you will have stirred the finish gently before you pour, to mix up those flatteners.
Choose a game plan. You will want to do only a swath of finish that you can conformably reach across, so if your are applying the finish by hand with a brush dont do more that a 3 foot by two foot area at a time. And if you are using that brush on a pole you can increase this to a 6 by 3-foot space. Of course each area will be one block in a series with the run with the floorboards. If this is parquet floor you will want your long swath of finish to be pointed at any windows, or parallel to a long wall. In either case keep the final brush strokes going always in the same direction for all coats. So, first wet the area by brushing in any direction, and then comb the finish once and only once in the chosen direction. You will have only brushed the finish twice, so have not created too many bubbles in the finish. You will have overlapped the previous area just a few inches, so as not to disturb this last patch too much either.
Do this slowly and deliberately, and if you are on your hand and knees (the best way to coat a floor if you have the really great Pro Knee pads). You should look at the light from the window reflected off the floor, this will show you what you really see what you have missed. Overhead lights, and coating a floor at night are not good at all. You need that angled light that only windows and strong sunlight can provide. I never start coating a floor past 2 PM. I just wait for the next day. Again Im trying to tell you the best way to coat of floor, and if you are impatient, and rush through this second coat, you will have a lot more work to do before the third and final coat is applied. And if you are production floor mechanic Im sure your eyes glazed over when you heard I brush on my finishes. Close the screen page now if you are too impatient to read this whole treatise. Your finishes will be forever bubbled and a pox on you.
Once you have brushed yourself out the door in a pre-planned route, let this second coat dry at least 40 hours. It will have been applied at the rate of about 500 -600 square feet to gallon. Keep track of how much finish you are using, too thin a coat, and you wont have enough build of finish to protect the wood. Too much and it will never dry properly. And thats the rub, this second coat has to dry well enough so that again, when scuff sanded (or screened with the buffer) the sandpaper or screen doesnt clog with bits of undried finish. In some cases in the middle of summer I cease to use OMP at all. I take most of August off and go fishing. In temperatures exceeding 80 F and humidity exceeding 75% OMP will just sit there like a pool of undried oil. What a mess that will be. This often happens on older floor that have been waxed or treated with various silicone or oil polishes.
The screening of the second coat is very critical step. You must not only scratch all of the floor surface, but all the little dust bits and bubbles that occurred in the second coat must now be buffed out, even if you have to sand some spots by hand. This is your last chance. Vacuum as before and tack rag, and be even more diligent when coating the floor this time, dont miss a spot, or create a blob in the finish. We used to call these holidays and silver dollars. My boss used to yell out the door as we were leaving the shop in downtown New Orleans. "By god, Boys, leave no holidays or silver dollars in the finish today". But if you got them in the third coat, either live with the silver dollars, or sand them flat and touch them up. You have to re-coat the entire board otherwise the finish edge will show.
Now, if you have followed my instruction to the letter and have used the Fabulon brand heavy-duty polyurethane satin finish, you will now have an almost bubble free floor. The only defects in it are so minor, that after a month or two they are abraded out of the top of the finish.
If on the other hand you have used only half my methods, and strayed into your old bad habits, rushing through the job, to get paid, or for the DIY ers just to get life back to normal, your results will be something less that spectacular. But I have a few more hints. Wear a hat when coating (we hate to see hairs caught in the finish), and be sure to wear clean clothes the day of the second and third coat. Ventilate all coats with a large fan about 4-5 hours after the finish is applied (or when the surface is hard enough not to have dust stick to it). You only need to clean the brushes when done with all coats, in the mean time keep them cleaned with paint thinner but wrapped in a thinner soaked rag. When you have completed the job, clean the brush until the thinner runs clear off the brush (thats what the roasting pans are for). And then wash your fine brushes with dish soap and warm water.
Oh, and I almost forgot, when doing kitchen floors add one more coat, the same would go for a heavily used dining room. And you will, in every case need to re-coat a kitchen floor every 2-4 years, and a normal used area, every 5-10 years. It just depends on how well you maintain the floor. In all cases if the floor looks good at the maximum times I just mentioned screen and re-coat it anyway. This is the only way to prevent water damage in the future. Well-coated sand on site floors will be more moisture stable, and will not form gaps in the dry winter months. All the methods I have mentioned so far are NOT for the pre finished floor. With all the little beveled edges, and the factory conversion finishes there is some doubt as to whether these floors will accept another coating of any type. There are new methods out there for recoating prefinished floors, but until I test then for adhesion, I will not write much about them.