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Spraying oil based urethane

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Forum topic by steveinaz posted 03-01-2018 01:56 AM 505 views 0 times favorited 11 replies Add to Favorites Watch
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steveinaz

46 posts in 1378 days


03-01-2018 01:56 AM

What is causing this? I am using an inexpensive Rockler HVLP system, there is only a volume adjustment on the gun. Shooting oil based urethane, thinned 10%, overlapping strokes 50%. I’ve shot this panel about five times (i’ve sanded it down between each test),working out technique issues and such, this streaking keeps coming back. Before I spent $600 to $800 on a good spray system I wanted to work out other issues. Right now I’m thinking, forget staining, forget topcoat, natural wood and Waterlux only. Any help would be appreciated.

-- Steve in AZ


11 replies so far

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Rich

3893 posts in 793 days


#1 posted 03-01-2018 05:12 AM

Are you doing a crosshatch pattern? It’s hard to say without more info.

-- Half of what we read or hear about finishing is right. We just don’t know which half! — Bob Flexner

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tomd

2175 posts in 3974 days


#2 posted 03-01-2018 05:45 AM

From what I think I see the panel is make up of edge glued boards. You may not have allowed enough time for all the moisture to escape and fully dry and that moisture maybe trapped under the finish. That is just a guess on my part.

-- Tom D

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John_H

188 posts in 1910 days


#3 posted 03-01-2018 07:59 AM

Not sure what exactly is going on from the pics – but it does appear to be evenly spaced and consistent with your spray pattern

Are you putting down a wet enough coat – what does it look light right after you spray it?

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EarlS

1943 posts in 2552 days


#4 posted 03-01-2018 01:37 PM

Judging from what I can see and guessing a bit, I wonder if the cloudy areas are a result of too much poly in those areas. If so, try a little less overlap and a lower volume setting. I don’t thin my poly (Arm-R-Seal usually) and spray just enough that there is a glassy sheen from the wet finish. It is always a challenge not to go back over the piece and put just a bit more on. Spray it and walk away, thin coats are better. If you aren’t getting coverage, then increase overlap or volume.

Another possibility with the “streaks” is that you are sanding off some of the stain when you sand between coats. I use 800 grit paper and lightly sand between coats. Sanding stained wood is always a balancing act since you need to get the nubs and such off without taking off stain. Re-staining after putting on a couple of coats of poly is difficult without completely sanding back to raw and and starting over.

-- Earl "I'm a pessamist - generally that increases the chance that things will turn out better than expected"

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jbay

2893 posts in 1103 days


#5 posted 03-01-2018 02:17 PM

Check the fan on your spray gun, maybe the tip is dirty applying more finish to one side of the fan.
It also looks like there could be moisture trapped under the finish. What’s the humidity? Temperature?
I also think you are over thinning.

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steveinaz

46 posts in 1378 days


#6 posted 03-01-2018 02:36 PM

It is not a cross hatch pattern. Immediately after application it looks great. If anything, I think the cloudy areas have less coverage?? I go for a pretty wet coverage and thin 10% to promote self leveling. I believe with the inexpensive gun I am using that the pressure at the center of the fan is greater than the edges and the streaks clear area had more pressure?? I sand 400 between coats.

-- Steve in AZ

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jbay

2893 posts in 1103 days


#7 posted 03-01-2018 02:43 PM

I mis read, I thought you thinned 50%. My bad.
What about humidity and temp?

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steveinaz

46 posts in 1378 days


#8 posted 03-01-2018 02:58 PM

It’s dry in Arizona, 65 degrees at time of spraying. This is a well cured panel, it has been in my shop about a year.

-- Steve in AZ

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Rich

3893 posts in 793 days


#9 posted 03-01-2018 03:20 PM


It is not a cross hatch pattern. Immediately after application it looks great. If anything, I think the cloudy areas have less coverage?? I go for a pretty wet coverage and thin 10% to promote self leveling. I believe with the inexpensive gun I am using that the pressure at the center of the fan is greater than the edges and the streaks clear area had more pressure?? I sand 400 between coats.

- steveinaz

Conventional wisdom says to do a cross hatch pattern — up and down, then back and forth. However, if your spray gun isn’t spraying evenly like jbay mentioned, it’ll probably still look uneven.

You can test to see if it clogged versus having a damaged tip by setting a wide fan spray and spraying a pattern on something that will show the pattern clearly, like cardboard or brown craft paper. If the spray is uneven from top to bottom (or side to side depending on you you have the fan directed) then rotate the spray fan control plate 180º and spray another pattern. If the heavy spray switches sides, you have a clogged spray head that you can take off and clean. If not, the needle is damaged and needs to be replaced. On an inexpensive gun, just replace the gun.

Doing a test spray is a good idea anyway. If you have uneven spray due to pressure or setup, it’ll show that, and you can tweak it to get it right. Even cheap HF guns are capable of a good pattern if set up right.

-- Half of what we read or hear about finishing is right. We just don’t know which half! — Bob Flexner

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pintodeluxe

5798 posts in 3017 days


#10 posted 03-01-2018 03:56 PM

Looks like the material is not flowing well out of the cap and nozzle of your gun.

1. Material may not be intended for spray applications (you didn’t say what product you used).

2. Material may be too thick for the needle / nozzle setup on your gun.

3. Tip could be partially clogged, which is related to #2 above.

Imagine good flow from the center of the gun, but restricted flow from the sides of the fan. That’s exactly what the end result would look like. Spray some test swaths on cardboard to check for a consistent pattern. Try additional thinner, and repeat. Clean the tip by soaking it in thinner.

Sometimes wet sanding will clean it up, but if not you may need another thinner coat.

-- Willie, Washington "If You Choose Not To Decide, You Still Have Made a Choice" - Rush

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steveinaz

46 posts in 1378 days


#11 posted 03-01-2018 04:10 PM

Varathane Ultimate Spar Urethane satin. I am going to check the flow from my nozzle as suggested. It seems to make the most sense to me. Thanks for all the help.

-- Steve in AZ

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