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Spray finish, what am I doing wrong?

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Forum topic by Tim posted 07-23-2019 03:16 PM 930 views 0 times favorited 26 replies Add to Favorites Watch
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Tim

46 posts in 3662 days


07-23-2019 03:16 PM

I’m spraying General Finishes QTHG High Performance Water Based Topcoat, using a Harbor Freight purple HVLP air gun. I have not thinned out the topcoat and I’m running the gun at 20-30 psi.

I can’t seem to get a smooth application at all, played with the fan control, fluid control and adjusted the air pressure up to 40 psi before going back to 20ish.

I’m about to drop the pressure a little and add some distilled water to the topcoat.

Thanks in advance.

-- Tim - Austin, TX


26 replies so far

View jmartel's profile

jmartel

8516 posts in 2600 days


#1 posted 07-23-2019 03:28 PM

Look up orange peel fixes. Thin it by 10% and try it again. Maybe bump up the air pressure.

https://www.woodshopnews.com/columns-blogs/orange-peel-can-leave-you-feeling-bitter

-- The quality of one's woodworking is directly related to the amount of flannel worn.

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waho6o9

8708 posts in 3027 days


#2 posted 07-23-2019 03:29 PM

jmartel for the win.

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Tim

46 posts in 3662 days


#3 posted 07-23-2019 03:56 PM


Look up orange peel fixes. Thin it by 10% and try it again. Maybe bump up the air pressure.

https://www.woodshopnews.com/columns-blogs/orange-peel-can-leave-you-feeling-bitter

- jmartel

Thank you, I didn’t realize this was orange peel I thought orange peel was related to contaminants.

-- Tim - Austin, TX

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SMP

1308 posts in 356 days


#4 posted 07-23-2019 04:13 PM

Whats the temperature and humidity when you are spraying?

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pintodeluxe

5970 posts in 3263 days


#5 posted 07-23-2019 04:24 PM

You’ll solve the issue with either thinning the product, or using more air to atomize it. The problem with cranking up the air pressure on a conversion HVLP gun is you increase overspray.

So the first thing I would try is thinning the topcoat. I usually spray lacquer, but sometimes shellac and poly. I thin the first coat by 20-25%. Then the second coat is thinned by 10-15%. I usually only need two coats.

Fisheye is the contaminant term you might have been thinking of.

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

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PPK

1470 posts in 1259 days


#6 posted 07-23-2019 05:02 PM

Be careful with thinning the waterborne finishes. I’ve been told that you can’t thin them more than 15%.
Ive gone up to 18-20% and ran into issues with blushing, running, etc.

My Harbor Freight purple gun runs best at 40 PSI. The knob at the end of the handle by the air connection can fine tune the air pressure if you’ve got your compressor roughly at 40 PSI.

-- Pete

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PPK

1470 posts in 1259 days


#7 posted 07-23-2019 05:08 PM

This may be bad advice, but I just finished a project with waterborne poly (Minwax brand) that didn’t turn out with a real great smooth finish. I handrubbed it with steel wool and paste wax, and then on the large surfaces used my random orbital sander with a polishing sponge. It turned out really good. Moral of the story is if there are dust nibs or minor imperfections in your finish, a lot of the can be rubbed out at the end. Probably not a 100% fix for what you pictured above though.

-- Pete

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Rich

4690 posts in 1039 days


#8 posted 07-23-2019 05:57 PM

Be careful trying to rub the finish to smooth it. Poly doesn’t amalgamate like lacquer does. Because it’s many separate layers you can get some ghosting if you cut through one layer into another. Think of a topo map. Same thing.

-- My grandfather always said that when one door closes, another one opens. He was a wonderful man, but a lousy cabinet maker

View Sark's profile

Sark

159 posts in 810 days


#9 posted 07-23-2019 07:01 PM

General Finishes sells an additive to help flow out…basically a retarder to slow the drying. My guess is that either the finish was too thick before spraying, or it was too hot in the shop and the finish dried too fast before leveling out. EF Extender its called (or used to be called). Distilled water can be used for thinning too.

Another possibility is a primer (if one was used) that is incompatible with the top coat. If no primer was used, then the culprit could be surface contamination or some incompatibility with the material you are trying to coat.

Did you prime before finishing?

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CharlesNeil

2495 posts in 4321 days


#10 posted 07-23-2019 11:11 PM

I’m not up to typing alot
If you will send me a PM
I can call you
I have shot a ton of HP
Glad to help

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CharlesNeil

2495 posts in 4321 days


#11 posted 07-23-2019 11:12 PM

I’m not up to typing alot
If you will send me a PM
I can call you
I have shot a ton of HP
Glad to help

View CaptainKlutz's profile

CaptainKlutz

1623 posts in 1944 days


#12 posted 07-24-2019 12:17 AM

I have the same issue with GF WB finishes here in AZ with high temp and low humidity. The alcohol solvents in WB finish evaporates too fast and doesn’t level well.

+1 look up orange peel solutions.
Usually more air, or lower viscosity using a slow evaporating solvent; solves the issue.

+1 Suggest the HF purple gun works better at 40psi, but it does increase over spray.

+1 Add GF brand Extender. Think it’s called Enduro extender these days?
It’s basically a propylene glycol and water mixture, or AKA slow evaporating poly alcohol. Can get propylene glycol from equestrienne or cattle supply, they sell gallons for $20 as medicine for live stock. You only use 5-10%, so gallon lasts forever.

Here in AZ I can not even think about spraying most finishes when temp is above 90F, due < 15% RH.
So I have to spray before ~8 – 9am in summer. When it’s hot and dry; the selection of solvent becomes important. I add retarder/extender and slow solvents to everything I spray. Straight water in WB finish tends to increase chance of runs, so I use slow alcohol/water blended. Try adding 10% by volume of 70% Isopropyl alcohol (livestock store again, or from pharmacy) and 5% GF extender to GF WB. Those solvents and little higher pressure will usually solve my summer time orange peel issues with low RH.

As always, YMMV.

Best Luck.

-- I'm an engineer not a woodworker, but I can randomly find useful tools and furniture inside a pile of lumber!

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diverlloyd

3560 posts in 2307 days


#13 posted 07-24-2019 01:48 AM



I m not up to typing alot
If you will send me a PM
I can call you
I have shot a ton of HP
Glad to help

- CharlesNeil

That’s a very nice offer you should take him up on it.

View OSU55's profile

OSU55

2380 posts in 2440 days


#14 posted 07-24-2019 04:05 PM



Be careful trying to rub the finish to smooth it. Poly doesn t amalgamate like lacquer does. Because it s many separate layers you can get some ghosting if you cut through one layer into another. Think of a topo map. Same thing.

- Rich

I’m pretty sure the GF HP is like Target EM9000 – if sprayed within 24 hrs the coats do amalgamate. (waterbased poly not solvent)

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OSU55

2380 posts in 2440 days


#15 posted 07-24-2019 04:13 PM

One of the issues with cheap spray guns is poor atomization, requiring higher pressure, which dries the finish faster. Thinning has some benefit but wb finishes are usually limited to ~10%. The extender products can really help this out by both thinning and slowing the dry time, allowing the finish to flow out. I’ll have to experiment with Capn Klutz’s recommendation of propylene glycol, a lot cheaper alternative. Read the instructions with the extenders – they need to be mixed in and then sit for a while.

You should definitely take up Charles’ offer.

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