Advice on spraying polyurethane - Woodworking Talk - Woodworkers Forum
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post #1 of 33 Old 06-05-2008, 04:59 PM Thread Starter
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Advice on spraying polyurethane

I am completely fed up with brushing polyurethane. I am obviously terrible at it because I canít seem to do it without getting runs or missed spots.

I am very meticulous and take my time to look over a project in close detail looking for runs and dry spots and never see any. Then I walk away thinking I did a good job only to return later to small drips, dry spots and waves. I re-sanded my entire project and this time made sure to only poly one side at a time laying the piece down flat. When it dried I had what looked like water spots over then entire piece. Perhaps I used too much poly I donít know. What I do know is that I am fed up.

The piece is a set of folding screens for my father in law measuring 6' x 5' and a coat of poly with light sanding takes me a couple hours. I really donít want to keep doing this. If the piece wasnít a gift I wouldnít even worry about the runs because you can only see them under a 1000 watt halogen work lamp.

I have a Husky gravity feed spray gun that I bought to paint my boat. I believe it should spray poly just fine. It worked great with the automotive paint that I used on the boat.

Does anyone have any tips or things to watch out for before I try this? I also have a book case that will need to be finished and I really donít want to brush that either.

Thanks,

David
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post #2 of 33 Old 06-05-2008, 07:57 PM
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spray finishes

David,
I don't like polyurethanes and haven't used any in a long time, but that's my own personal thing. I like quick dry varnishes. I spray these and have very good luck with them. I have several HVLP guns, but the one I use most is an automotive trim gun that I bought at Lowes for about 39.00. It has a trigger lever on top and a small canister below that is about the size of a small orange juice concentrate can. I thin the varnish with paint thinner at a 4 parts varnish to 1 part thinner. You can adjust that gun to spray from about 1" wide to 8" wide. I spray cabinets, moldings, etc. The varnish is dry to the touch in about twenty minutes. After the first coat I lightly rub it with 0000 steel wool. This smooths out any fine bumps from the grain raising. Vac and tack rag and respray. I usually spray 4 coats and it comes out waxy smooth. I usually set the material adj. knob so that I have to make 2-3 passes to get a wet coat. Experiment on cardboard first. This gun only requires 35-40 psi and doesn't result in a lot of overspray.
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post #3 of 33 Old 06-05-2008, 09:54 PM
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I have sprayed quite a bit of oil base polyurethane, and don't like to do it. If you have to, deliver very thin coats. It's a heavy bodied material that stays wet too long.

Other than brushing it, you can wipe it on. Thin first coat or two with mineral spirits. When each coat has cured, use full strength. You can get a very good finish that way.

Just for grins, try WB polyurethane. It's easy to use, dries fast, looks nice.






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post #4 of 33 Old 06-06-2008, 07:41 AM
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Just a thought.... if you purchase a HVLP sprayer, I might suggest using water and some cardboard to practice on, saves a lot on wasted product.

I have a HVLP spray gun as well and for me is a lot better than the brush, mind you I have seen some people with brushes that make my spaying look weak.

Good luck, finishing does make the project eye appealing.

Have a great woodworking day

John

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post #5 of 33 Old 06-06-2008, 09:34 AM
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I've done it once - sprayed oil-based poly with an HVLP gun. It turned out OK but was a hassle. Thin it down a lot (sorry, I don't recall the ratio but at least half or more thinner). That helps you to spray thin coats and helps it dry faster. If you get a run or sag (and you will), use a razor blade to scrape it off and hit it again with a very light coat. It worked for me.

Actually, I find that the poly in aerosol cans at Lowes or Home Depot works great for a finish coat. I hate to buy it in the aerosol cans because it's very expensive (you're paying mostly for propellant and not product) but it works well for a final coat.
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post #6 of 33 Old 06-06-2008, 10:47 AM Thread Starter
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Last night I lightly steel wooled one of the panels and rebrushed on some poly. My brush had been sitting in some mineral spirits since the last coat. I shook out the spirits and used the brush to stir the poly. There was about a quarter of a quart can remaining. The reason I said all that was because I donít know if the fact that I didnít completely dry the brush is what caused my next issue.

The coat went on very nicely, nice and smooth and the brush carried the product very well. I am guessing because the brush has some spirits in it. When the poly dried, some of it looked good and some of it had what looked like streaks on a window that was cleaned with 409 instead of Windex.

I donít get it because I took my time and brushed very slow with even stokes and a nice thin coat.

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post #7 of 33 Old 06-06-2008, 11:03 AM
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I am not near good or the most patient finisher. Just wonderer about the poly, how old is it and did you get the spirits all out of the brush you used.

These may play in the finish, as well tempature plays a role in the drying. Did you lightly sand between the coats. Did you use the proper type of brush.

Now you see why I shy from brushes, they just don't like me.

Good luck on your finishing.
John

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post #8 of 33 Old 06-07-2008, 08:29 PM Thread Starter
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There was still mineral spirits in the brush. I think that is what caused the streaks. I thought it would be ok but I guess not.

Ok so I went out and bought a new can of poly (because mine was old), 2 aerosol cans of poly, and a can of rub on poly. I intended on trying my spray gun, the cans, and the rub on if all else failed. I just got finished with the aerosol can and my god what a difference. The only imperfections were left over from the brush that I didn’t sand out well enough. I’m amazed that a spray can did better that a 12 dollar brush. I had more in the brush than I did the poly.

David
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post #9 of 33 Old 06-07-2008, 08:34 PM
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i never tried spraying so i am not bashing it but when i use poly i buy miniwax's rub on poly and it works great

~Jake Mendez
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post #10 of 33 Old 06-07-2008, 09:03 PM
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David

I'm glad you are making a little headway with your finishing.

I have never used the rub on Ploy, but I may give it a shot on the next project.

Have a great day
John

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post #11 of 33 Old 06-08-2008, 01:16 AM
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After years of brushing on poly, I began spraying about 2 1/2 years ago. I spray a satin polyurethane, thinned 10% by volume, with an hvlp sprayer, at 35-40 psi. I spray light coats and rarely get drips, runs, or sags. I get a beautiful finish out of it. I typically apply 3-4 coats. Have your surfaces horizontal as much as possible.
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post #12 of 33 Old 06-09-2008, 01:17 PM Thread Starter
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The spray with can is working wonderfully. I need to read up on some spraying techniques though because what I am finding out is that as I spray one part, particles are settling on other parts. Its no big deal because the particles rub off but I imagine that there is a process for avoiding this. Looks 100 times better than my brush work.
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post #13 of 33 Old 06-09-2008, 03:17 PM
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djonesax, another option is to use the foam brushes. If I'm reading your post correctly, you're using good old fashioned brushes made from some hair or another. I've had very good success using the foam brushes and getting a very light, even, coat with no runs.I've also found heat helps. I have some shop lights that are 500 Watt halogens that really heat the surface up so when I need something to dry quickly, I blast it with two of those and it really helps prevent any runs.
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post #14 of 33 Old 06-10-2008, 11:36 AM Thread Starter
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I have used foam brushes to apply stain but never for poly. I thought about it but never tried it. I am very pleased with these spray cans though. There is a $40 small kobalt spray gun at Lowes that is the perfect size for most of my projects. I was going to pick it up last night but forgot my wallet and had to put it back. :-( It had a cup that I could position different ways for different spraying angles.

Does anyone have any ideas for a portable spray booth? I was thinking I could make some plastic panels out or 2x2 that latch together somehow. Something I could tear down and put up easily.
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post #15 of 33 Old 06-10-2008, 01:21 PM
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" Does anyone have any ideas for a portable spray booth? I was thinking I could make some plastic panels out or 2x2 that latch together somehow. Something I could tear down and put up easily."

There are many booths but I will caution you on explosions. If you use a fan you will have to find an enclosed fan for that purpose. If you are indoors it should be ventilated outside. I open my doors and windows in my shop wear a respirator and try not to do a lot at the same time. This is very inconvenient but I do not have the space for a proper spray booth. Even if you were to use water based poly, the odor is quite strong and has the potential to be dangerous. The lightingt as well should be approved for spay booths as well.

In my opinion either do it right or do it small.

John

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post #16 of 33 Old 06-10-2008, 07:36 PM Thread Starter
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OK don't shoot me but I was considering using a camping tent. I could get an old tent large enough for me to put my piece and be able to walk around it. I could spray during the daylight out in the yard. Sunlight typically will penetrate a tent wall. Then some how rig up a fan system. I would imagine that the problem with having a fan blowing out would be the need to have fresh air to come in and that could bring in unwanted particles. I guess I could use a couple of accelerant safe box fans (if they even make such a thing). With the one on the inlet blowing through a cheap air filter. I worry about spraying in the garage because there is alot of stuff in there and I dont want to get paint and poly all over everything. Actually I dont really care, it's that wife that freaks out.
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post #17 of 33 Old 06-10-2008, 08:10 PM
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Man can I relate, LOL,,,, "Actually I dont really care, it's that wife that freaks out." I am always looking for ways to keep under my better halfs radar.

Try this for some info, these 2 sites may shed some light.

http://pages.interlog.com/~ask/scale/tips/booth.htm
http://www.ehow.com/how_2067309_build-spray-booth.html

Good luck on your mission
John

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post #18 of 33 Old 06-10-2008, 10:19 PM
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spray booth

Dj,
Nix the tent idea. I spent 25 years in the fire service, and I remember on particular house fire where a painting crew was refinishing a first floor den that was all wood panels. They were spraying lacquer with a box fan in one of the windows blowing out. Didn't take long for the room to flash. We arrived in a just a few minutes and the front, middle of the house was on fire, two floors. We put it out in short order, but needless to say, there was quite a bit of damage and luckily no serious injuries. If you want to make a simple booth for spraying out in the yard, get a few blue tarps or visqueen and some pvc pipe. Make the booth with three sides and the top enclosed. Point the open side downwind and spray away. You should still wear a good quality respirator that is approved for the materials you are spraying. If you are using quick drying materials, you shouldn't have a problem with dust settling. I have done this to spray a few cars and they came out pretty decent. It works good for spraying varnish also.
Mike Hawkins
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post #19 of 33 Old 06-12-2008, 02:44 PM Thread Starter
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post #20 of 33 Old 06-12-2008, 03:06 PM Thread Starter
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Ok so for sure the tent was a bad idea.

I read a quote by Einstein once that read "As our circle of knowledge expands, so does the circumference of darkness surrounding it. ". So in other words, the more answers I get from you all, the more damn questions I have.

I made a bookcase for my wife a few months ago which turned out good and now my mother wants one. For the first bookcase, I used a brush to apply the poly. For obvious reasons I want to spray the next one. I’ve sprayed a 15’ boat with my HVLP and many interior walls with my airless sprayer. I’ve always read that you should keep a consistent distance, steady pace and be parallel to the work. The shelves are about 14 inches apart so getting a gun inside and spraying parallel to the shelves will be difficult. Should I stain and finish the insides prior to assembly?

Also do you all recomend any type of finish that will add some extra resitance to dents and scratches? Polyurethane doesnt seem to do much. The book case will be pine and I would like to try to make the wood more dent and scratch resistant.

David
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