# Spraying Polyurethane



## Gary R (Jan 11, 2012)

Don't know what I am missing here but there is surely something.

I am using a small HVLP gun with a 1.0 needle that sprays catalized acryllic urethane clearcoat [automotive] beautifully but just cannot get any decnt reuslts when using the same proceedure with poly.

I am spraying at 25 to 40 lbs and using Minwax gloss poly. I have tried it full strength and also thinned with naptha and also with mineral spirits. Get nothing but runs and a really poor covering film, just does not want to level to a nice finish.

Could anyone give me a clue as to what I am missing? This should be a simple thing but is giving me fits.

Thanx..............gary


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## cabinetman (Jul 5, 2007)

Oil base varnishes and polyurethanes are heavy bodied media. They are difficult to spray as it's usually applied in excess. Surfaces may not appear covered, but are, so further application is dispensed, leading to runs and sags. It has a slow "set" time, which is long enough to allow the problem.









 







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## Modela (May 27, 2012)

I spray urethane regularly with a conventional gun. Sometimes I thin it with a small amount of paint thinner (mineral spirits). I have been doing it for years with a devilbiss JGA gun with good results.

I haven't tried it yet with my HVLP gun. Like you said, the clear coat automotive paint works great with an HVLP.

I have been told that if you crank up the pressure on and HVLP gun it works the same as a conventional gun but I have never tried it.

Jim


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## Steve Neul (Sep 2, 2011)

Gary R said:


> Don't know what I am missing here but there is surely something.
> 
> I am using a small HVLP gun with a 1.0 needle that sprays catalized acryllic urethane clearcoat [automotive] beautifully but just cannot get any decnt reuslts when using the same proceedure with poly.
> 
> ...


On oil based poly or varnishes you thin it as little as possible. In hot weather I would use mineral spirits to thin it. The hard part is the thought of spraying it. With your automotive finishes you spray a full wet coat. With an oil based poly, visually when you spray it, it should look like you would get a major case of orange peal. As it is being sprayed it should have somewhat of a texture to it. It is not likely to orange peal. It dries so slow the texture will flow out after you walk away from it. If you thin it to where it sprays well then it ruins the finish like you are describing. If you put enough on that it eliminates the texture than it runs on vertical surfaces. It also makes it take too long to cure properly. Try this on some scrap wood first before you use it on your project.


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## Cpcphil (Jul 3, 2012)

I'm not a big polyurethane fan especially if you are used to spraying automotive grade clears. Spray a precatalyzed lacquer or a conversion varnish instead. The viscosity of these materials are closer to the automotive clears so your hvlp gun with a 1.0 needle should spray just fine. If you continue to use polys then get a 1.2 tip for spraying thicker viscosity materials like the poly. 

Good luck


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## Gary R (Jan 11, 2012)

The reason I am trying to spray poly on these gunstocks is because there is a rejection of the auto urethane to every barrier coat I have tried. If I could come up with a sealer barrier coat that would meld with the auto urethane then I would not be fooling with the poly.

SW has a system but it is costly and complex so I am not a player.

Anyone know of a barrier coat/sealer that will dig into the wood and still provide a good base for the automotive catalized urethanes?


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## Steve Neul (Sep 2, 2011)

I don't know of any reason you can't put an automotive urethane clearcoat directly on the wood. Granted the sanding between coats would be harder do to the hardness of the urethane but it should work. Kwick Kleen makes a exterior fast dry polyurethane that works more lake lacquer. It melts into the previous coats like lacquer does. Being fast dry it sprays better than convential Poly.


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## Gary R (Jan 11, 2012)

Steve...... automotive catalized acryllic urethane definitely will not adhere to wood. Been there/done that. Chips, delaminates and peels like crazy.

The secret has to lie in the correct barrier coat. Thought de waxed shellac would do the trick but nada. Works great with poly though.


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## Gary R (Jan 11, 2012)

Cpcphil said:


> I'm not a big polyurethane fan especially if you are used to spraying automotive grade clears. Spray a precatalyzed lacquer or a conversion varnish instead. The viscosity of these materials are closer to the automotive clears so your hvlp gun with a 1.0 needle should spray just fine. If you continue to use polys then get a 1.2 tip for spraying thicker viscosity materials like the poly.
> 
> Good luck


 
Here I go showing my stupid again but..........
what is a conversion varninsh?
is it a good finish for a fine gunstock?
where do I purchase said c/varnish?

Thanx.


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## Steve Neul (Sep 2, 2011)

Gary R said:


> Steve...... automotive catalized acryllic urethane definitely will not adhere to wood. Been there/done that. Chips, delaminates and peels like crazy.
> 
> The secret has to lie in the correct barrier coat. Thought de waxed shellac would do the trick but nada. Works great with poly though.


 I've only used automotive urethane once on wood a long time ago and I don't remember which product I used but it worked. It was a gallon someone gave to me. I think it was Nason but it has been too long. I used it on walnut that wasn't stained and I did not seal it with anything. I just sprayed the urethane directly on the furniture. Since then I've seen several people on a autopaint forum talk about using urethane on wood with good results so I thought it was a accepted practice. The only thing I can think of is the urethane that you are accustom to working with was too hard for wood and it didn't go along with the expansion that wood does. Perhaps a different brand would have different results. At one time I used Dupont furniture lacquer exclusively and I quit because it would crack like glass. I personally think any automotive paint is too expensive to use on wood.


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## Modela (May 27, 2012)

I built this rocking horse using red oak over 20 years ago, sprayed it with 3 coats of urethane using a Devilbiss JGA gun. Besides our kids use we have loaned it out to others. No runs, a great hit and no errors.

The finish on the cherry wood is Sherwin Williams catalyzed lacquer which is supposed to be almost equal to urethane. It actually did pretty well but I don't think it was as good as urethane. Over all these years Urethane has been my product of choice.


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## JCCLARK (Jan 30, 2007)

I have sprayed many wood projects with automotive urethane
and never had a problem. It's the most durable finish out there.
If it peels, you've done something wrong.


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## Cpcphil (Jul 3, 2012)

Just to answer your question about conversion varnishes... It is a relatively quick drying coating used where you need excellent resistance to moisture, chemicals and abrasion. We use this type of coating on bar tops, laboratories, restaurant booths and tables just to mention a few areas. The down side is that this catalyzed coating is not very flexible after it cures which makes it vulnerable in repeated freeze thaw cycles is in temperature extremes of continued exterior exposure. 

It applies very similar to exterior urethanes. Dries much faster than poly, can be self-sealing, comes in clear and pigmented versions and will lay down like a lacquer. 

Also for your barrier coat on wood you should be able to reduce your first coat of urethane at least 25% and spray a light fog (tack coat) on your bare wood, let that flash off then apply a full wet coat then proceed with your additional coats. Urethanes are very temperature and humidity sensitive so make sure you keeping track of them during application. Also many urethanes have a critical window meaning that you my have to wait a certain amount of time between coats but cannot wait too long without resanding prior to applying additional coats. Always follow manufacturer recommendations. 

I have also used a catalyzed vinyl lacquer sanding sealer as a barrier coat over stain on a wood surface prior to topcoats of urethanes as well. So there's another option for you. 


Hope this helps.


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## Modela (May 27, 2012)

Cpcphil said:


> I have also used a catalyzed vinyl lacquer sanding sealer as a barrier coat over stain on a wood surface prior to topcoats of urethanes as well. So there's another option for you.
> 
> 
> Hope this helps.


I have been told that Polyurethane does not stick well to sanding sealers. Are you talking about a more sophisticated one. Also, I have seen quite a difference in Polyurethanes. Which do you like best?

Thanks, 

Jim


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## Cpcphil (Jul 3, 2012)

Hey Jim,

When I was speaking about a barrier coat I was referring to a coat in between the wood and your urethanes that you are used to spraying. Lacquers and traditional polyurethanes do not like each other. They will bubble and have very poor adhesion to each other. 

I believe that what type of equipment, spray area and size of project all play into recommendations that I make. For example if you limited space, poor ventilation and temperature fluctuations then a quick drying lacquer sprayed through an HVLP gun makes sense if you can create airflow in your shop. If not then water based lacquers make sense if you can get temperatures to a consistent range of at least 65 degrees or above. 

It's hard to catch different fish if you always use the same bait...

I however am not a big fun of minwax products I general because they tend to dry too slow for any type of production applications. Having to wait up to 48 hours for a stain to dry is just a bit long for me. It could take up to a week to finish even a small project... Too for me to wait to get average results. 

I have done a lot of work with water based finishes over the last 10 years and I'm a really big fan of all the finishes from Target coatings.


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## banjopicks (Jan 3, 2009)

Cpcphil said:


> Hey Jim,
> 
> For example if you limited space, poor ventilation and temperature fluctuations then a quick drying lacquer sprayed through an HVLP gun makes sense if you can create airflow in your shop.


That goes against everything I' learned about spraying lacquer. Lacquer is highley flammable and should only be sprayed with an explosion proof fan running. So if you have poor ventilation, forget about lacquer or spray it outdoors. That works well for me because it dries so fast you don't have to worry too much about bugs.


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## Modela (May 27, 2012)

banjopicks said:


> That goes against everything I' learned about spraying lacquer. Lacquer is highley flammable and should only be sprayed with an explosion proof fan running. So if you have poor ventilation, forget about lacquer or spray it outdoors. That works well for me because it dries so fast you don't have to worry too much about bugs.


I have a spray booth in my shop with an explosion proof fan (5 hp). I agree about the explosiveness of lacquer. There is also the overspray problem with anything you put on.

We used a catalyzed acrylic lacquer by Sherwin Williams on the inside cherry of our house. It was supposed to be as tough as urethanes and it has stood up well over time. 

I have sprayed a lot of different kinds of paint. One was a panel I replaced in my motor home. It came pre-primed. I matched the paint and put on a low VOC clear coat (Nasun, I think). It went on beautifully with a HVLP Devilbiss gun.

My biggest problems have been finishing wood where stains and clear finishes just seem to get in the way.

I am really glad to be a part of this discussion. I am learning a lot.

Jim


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## cabinetman (Jul 5, 2007)

Modela said:


> I have a spray booth in my shop with an explosion proof fan (5 hp). I agree about the explosiveness of lacquer. There is also the overspray problem with anything you put on. Jim


If you have a booth, and it's efficient, your overspray should get evacuated.









 







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## Cpcphil (Jul 3, 2012)

banjopicks said:


> That goes against everything I' learned about spraying lacquer. Lacquer is highley flammable and should only be sprayed with an explosion proof fan running. So if you have poor ventilation, forget about lacquer or spray it outdoors. That works well for me because it dries so fast you don't have to worry too much about bugs.


Not all lacquer is flammable. Also using an HLVP on a small project such as a wood plaque is different than spraying a 10' conference table. I have also seen spray booths spontaneous combust as a result of lacquer dust and a spark so the issue is not just lacquer fumes but dried lacquer dust as well. I have also had more issues with rags that have been used for staining combusting on there own. 

Bottom line safety in the coatings world is not just the flammabilty of lacquers but the safe use and practice of stains, sealers and topcoats reguardless of brand. Respirators, gloves, etc all play into safe use of these type of products.


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## Cpcphil (Jul 3, 2012)

Modela said:


> I have a spray booth in my shop with an explosion proof fan (5 hp). I agree about the explosiveness of lacquer. There is also the overspray problem with anything you put on.
> 
> We used a catalyzed acrylic lacquer by Sherwin Williams on the inside cherry of our house. It was supposed to be as tough as urethanes and it has stood up well over time.
> 
> ...




Hey Jim,

Technique could be an issue here. In a cross flow booth the air is moved across your object that you are spraying. So when spraying start with your gun the farthest away from the fan working your way across the substrate toward the fan so that the overspray will be sprayed over with a full wet coat. 

Also narrowing your fan width to match the width of what you are spraying works well. If you are spraying legs of a chair then have a 6" in fan is a waste of material and increases your chance of overspray.


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## Modela (May 27, 2012)

Right, mine is a rear vent paint booth. For practicality reasons I left the front end open. The 5 hp fan clears the whole thing really well. I also coated the whole booth including the low temperature lights with a peelable clear paint booth finish--amazing stuff.

Jim


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## Modela (May 27, 2012)

I just finished a door using a clear catalyzed epoxy primer. The instructions were to apply the first coat by brush, not spray. The reasons not to spray were that the thinners evaporated. Then, let it dry for two days and sand. Following that apply a next coat and let it dry for one day.

The next step was to apply the finish coat (catalyzed urethane in my case) to bond to the not yet cured clear epoxy. Automotive urethane would have probably worked over the epoxy primer.

It seemed to work well.

Jim


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