Applying oil-based polyurethane

      Brushing and spraying problems and options. August 25, 2003

Question
Anybody had luck spraying oil based polyurethane? I've got a conventional spray gun (not HVLP) and I'm not having any luck at all trying to spray. I've tried thinning with mineral spirits and turpentine and still can't get it to look worth a darn. I tried brushing, even bought a $37 badger style paint brush. I got too many hairs from the brush on the finish. I got better results with a $10 white bristle brush. Still have brush marks, though. An article in FWW suggested thinning with acetone, but haven't tried that yet.

Forum Responses
(Finishing Forum)
To brush poly without brush mark problems, thin at least 10% from the can and don't over-brush. But better still, why don't you make your own wipe-on with a 1/1 ratio of poly and mineral spirits and wipe it on using those blue or white paper towels? Just fold the towel up in a pad like a handkerchief and apply using half of the pad. Again, don't over-wipe; just wipe on and then give a smooth wipe and move on to let it level itself. Practice briefly to get the knack. I can do table tops without brush marks or dust problems. Also, with wipe-on poly you can apply another coat without sanding in about 1-2 hours (dry to touch). Wipe-on is thin coats, so it may take 4-6 coats depending on the look you want.



From contributor D:
Why bother? That's why God invented lacquer and conversion varnish. Why use this lame material when stuff that's so much better is readily available?


The above post is right, but you can spray poly successfully by thinning about 15% with naptha and applying thin coats.


From contributor C:
Lacquer is nowhere near the finish that polyurethane is. Conversion varnish is a high quality finish, but it's toxicity demands a fully equipped shop for use. I often use polyurethanes for their durability and low toxicity and also for their compatibility with special effects techniques that I often use. HVLP is a great help in getting it to spray well and all polyurethanes are not equal. I like the Minwax products, which are (fortunately) widely distributed. I often spray them right out of the can or very slightly thinned with VM&P naptha.

A big disadvantage of polyurethanes is their slow drying and the sticky overspray that is a consequence. This is often a problem when using conventional spray equipment. The overspray fogs the previously sprayed areas. I do a fair amount of refinishing on existing cabinetry (mostly pretty high-end stuff) and I can tell you that polyurethanes will be tougher than most of the finishes that I replace and in most cases more chemically resistant too. For a professional shop, the slow drying can be a deal breaker, though. I tend to use it only when it has some qualities that I need. I won't spray conversion varnishes in an occupied home, for instance. I can brush on polyurethanes where needed with no significant loss of quality. I do some hand rubbed techniques where polyurethane is the only thing that works for the effects I am after.



What the above post says about polyurethane is true. Slow drying and therefore receptive to trash and other nits flying about. But what about subsequent repairs on poly? I find it very difficult to do so, unless the whole piece is stripped and done over. How about the yellowing effect with poly? Just some things I consider when I re-do a kitchen.

Personally, on site, water base polys should be the way to go... even spraying with proper masking and ventilation.



Try thinning poly with toluene (works with most). If not, buy a quality urethane reducer at an automotive paint store and that should work. Acetone will flash too quickly, leaving a lot of overspray.


From contributor C:
I do patched-in repairs on poly all the time. I hand rub the patched area to blend to a very fine feather edge, then I sand lightly with 400 grit and recoat the whole piece involved. The only applications I have had difficulty with because of the yellowing effects were when I attempted to use a clear coat over white or near-white finishes. I now use Breakthrough water base cross linked acrylic for such applications and prefer to use tinted material rather than to coat over a light base with clear.


From the original questioner:
Contributor D, I do use lacquer on things that are not subjected to water, such as beds and chairs and I get really good results. This time, I'm finishing an 8 foot dining room table and I want something that will stand up to water and other abuse. The brand that I am using is Minwax. I've tried both gloss and semi-gloss.


From contributor C:
Contributor J, when using gloss or semi-gloss, I usually do a rubbed out finish. For off the gun finishes, the satin is much more useful. A dining table such as you describe is a tough test of your skills. Do a careful job of between-coat sanding. It usually takes me from 6 to 8 coats to get a near perfect surface on the top of such a project. I have done it by brushing, but the sprayer is faster and does a better job, too. Spraying is a fairly complex skill, too, so you may want to do some reading and some test work. You still have to work out the surface prior to the application of each coat if you want a very high quality result. Spraying will cut down on some of the work (when compared to brushing) but full attention is still required.


I have sprayed Minwax poly successfully several times without thinning. I use a Graco 4-stage turbine HVLP with a 2 mm tip. Each time it went on nice and flowed out nice.


The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor A:
It might be simpler than you thought. My method works on nearly every type finish including poly. Read the product label and thin with whatever it tells you to use for clean-up (we'll call it thinner). Add thinner and mix well with a stir stick. Lift the stick and count... one thousand one, one thousand two, one thousand three, one thousand four. The finish should stop running off the stick and begin to drip. If not, add more thinner. This method has worked great for me.



Comment from contributor B:
I think poly is a better long term finish if you are patient enough during the application phase and your project is suited for the amber effects caused by it. I'm in the process of finishing interior trim and doors in my new house. I'm spraying Minwax poly using a Devilbiss HVLP gun. I'm thinning it with just mineral spirits at a ratio of about 6 parts poly to one part thinner. My biggest problem is the sticky overspray. HVLP guns are better than conventional on overspray, but you still have a significant amount of it. Since this is new construction, I'm also fighting the dust issue as well. There's no doubt that spraying has saved me a huge amount of time over brushing everything. I've done over 1300 lineal feet of baseboard / casing and 12 interior doors and frames in a little over a week's time. This would have been impossible with a brush!


Comment from contributor D:
Something not mentioned in any of the comments for using poly - the ancient art of scraping. On a large surface such as a table top, I have achieved excellent final results by lightly scraping with a single edge razor blade after the poly has dried for 8 hours between coats. It takes care of all the bubbles, dust particles, etc. and doesn't change the surface like sanding. I have even successfully scraped the final coat on oak tables, with no further need for rubbing - on satin finishes.


Comment from contributor M:
I do a lot of furniture refinishing, mostly of good antiques on which the original finish was spoiled or unsuitable. I never spray, because I am working in my home, and there is only so much stink we can stand.

I have excellent results using a paint pad to apply oil-based polyurethane on flat surfaces. You can use a paint pad just like a brush, but it goes much faster and goes on more smoothly.

Some paint pads, like some brushes, leave little hairs in the poly. I just leave them alone and sand them out when the poly dries. If a pad starts to shed, I cut a piece of clean, lint-free fabric and tie over the pad. I have little trouble with Padco paint pads shedding.

The best thing I've found for sanding between coats of any clear finish is a manicure block called Polar Block. Is is used to smooth acrylic fingernails. It's white, and about 4" long and 1" on each side. It has a very fine, sharp abrasive, cuts fast, leaves the surface very smooth, and does not load up. There is a finer manicure buffer that's yellow, and will leave the surface almost polished. Both blocks will get into tiny corners and work on curved surfaces. You can get them at most beauty supply shops.

Right now, I'm working on a Victorian walnut dining table, 52" square, that opens to 120" with 6 leaves. The surface of the top was in bad shape, and I've got 10 coats of poly on it and am ready to do the final sanding and top coat.

If I get dust in the final coat, I'll sand it with the Polar Blocks and polish with the yellow ones, and then buff the gloss up to where I want it with auto-body or metal polish. My dad used powdered pumice and motor oil, but I can get a smoother finish with the commercial polishes.

To use a paint pad, thin the poly with mineral spirits. I am in a high, dry area, and thin 4 parts poly to 1 part spirits. In a humid area, you might not need as much mineral spirits. Put the thinned poly in a flat pan. A disposable aluminum pie or cake pan works well. Dip the pad in the poly until its surface is covered, and stroke smoothly from the center of the piece to the edge, first one way, and then the other. Make another smoothing stroke the entire width of the piece, and go on to the next stroke.



Comment from contributor C:
To get good results with poly I apply 2-3 build coats straight out of the can. I sand between coats to knock down dust nibs and over- spray. After sanding the final coat I apply a coat of 50-75% poly and 25-50% mineral spirits "wiping poly" which will dry very fast and dust free.



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