Help me spray a better coat of waterborne finish

[Denis Kenzior]

I've been trying to teach myself how to spray finishes and quite frankly it has been a struggle. I can't seem to get a decent sprayed coat even under ideal circumstances. I've gotten by with rubbing out finishes but it is a lot of work and can be justified only for those special pieces. I'd like to get a bit more production minded and hopefully get some pointers from the more experienced finishers here on how to get a better finish.

So lets start with the setup first. Compressor is a 2 HP California Air tools, 10 gallon compressor. Putting out 6.4 cfm at 40psi. I've modified it to add a water trap / air filter with a dedicated hose for the spray gun.
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I use a makeshift booth with a 20x30 in box filter. The walls are just plastic sheeting hanging from the ceiling and encloses and area around 12x12 feet. The exhaust is drawn outside using a 12" diameter flexible hose and a 12" utility blower rated at ~3000 cfm. I've measured the airflow with a velocimeter and get around 120-150 ft/min at the filter. There is a dessicant filter at the end of the 15" hose and a final 3 foot hose to the gun. I use a cheap lazy susan bearing screwed to two pieces of MDF as a turntable.
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The gun for these trials is an Asturo WB with 1.5mm tip and RP aircap. I also have a 1.8mm setup, and both H1 and H2 HVLP aircaps available.
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I'll be spraying General Finishes Enduro PreCat Waterborne Lacquer. The results I get are pretty typical and are similar to what I've been experiencing with Target EM6000, EM9300 and GF Clear Poly.
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So on to the test piece. This is project I'm working on that I'm planning for a fully leveled, rubbed out and polished out finish. Wood is Mahogany. I applied 2 light sanding sealer coats and 3 full strength coats of lacquer by brush. Then sanded back using 180 grit to fill in the pores. Here's the result:
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Here's a sprayed coat:
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The surface is full of dimples, craters, popped and unpopped bubbles. Once dry the overall feel is quite smooth, but the craters and bubbles are quite noticeable. Since the finish is smooth, it doesn't appear to be a dry spray issue. Any suggestions on how to obtain better results?

[John TenEyck]

GF's PreCat Lacquer is not nearly as easy to spray for me as Enduro Clear Poly, so I would switch to Clear Poly.

If I were trying to get a filled, smooth finish on mahogany I would use a grain filler, like Crystalac, then spray a sealer coat of Sealcoat shellac, and then start with the Clear Poly. Two light coats, sand flat, then one wet coat should do it. Your 1.5 mm N/N should work fine.

John

[Robert Cherry]

Jeff from target coatings has told me that without a grain filler you will never eliminate the pinholes on mahogany using a WB product. I used a product called AquaCoat, which is about the consistency of petroleum jelly and troweled onto the piece after your seal coat, then sand. Worked well for me on a dining table top in sapele.

[Denis Kenzior]

John,

I agree that Clear Poly lays down nicer, particularly in lower sheen versions that also hide some of the imperfections. However, PreCat has burn-in qualities and I'm planning on rubbing this out. I also find that PreCat sands way better.

I have not tried Crystalac, but have tried AquaCoat which seems to be a very similar product. Not impressed. The stuff shrinks back like crazy. I've done 5-6 applications with a credit card wiping across the grain on mahogany and still had grain showing. Doesn't sand that well either. I've had better luck with Timbermate, but it changes the color of the wood a bit too much for my liking (especially if going for contrast) unless you sand it well back which exposes new pores, but does fill the really deep ones.

This is the look I'm going for (you might remember this one from earlier):

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That took me about 10 or 12, after using AquaCoat and mostly filling the grain. The pores on that board are nowhere quite as deep as on the veneered ply in the first post.

[Wayne Lomman]

If you want full finish, you will need a grain filler. This is rarely achieved using the coating alone but can be done using a good 2 pack polyurethane. This will be solvent borne so may not fit your requirements.

The other way is to apply a grain filler. My preference is to use a one pot polyurethane filler. You have to do some work with colour. You also have to do some work applying it by hand. However, rubbed in well and finished off with the grain leaves a surface that only needs a light 320 sand before the first coat is applied.

The open grain and popped/unpopped bubble issues will cease to be a problem after filling. The orange peel looks like a combination of product characteristics and viscosity. I would've inclined to thin the product a little more and play with your fluid and air controls to get better atomisation. Also expect to have to sand the finish flat before final coating.

If your budget will stand it, your booth filter needs to be several times larger. Ideally you need a full wall but if you can add at least 3 or 4 times the area it will help. You are working in a bit of a concentrated gale I suspect. Gun, compressor and traps are fine. Use the 1.8 however. Cheers

[Denis Kenzior]

Wayne,

My findings seem to be that the finish bubbles / pinholes regardless of whether the grain is filled or not. The board I sprayed was pretty much completely filled. Maybe a dozen tiny glossy specks from unfilled pores on a 18 x 14 inch area. I've tried spraying pre-finished birch ply and get the same result.

I could be completely wrong, but I don't think it is an atomization issue either. The spray pattern is actually really tiny droplets, but they don't flow out well. The product is meant to be sprayed at 3-5 mil wet thickness, but I spray at around 1-2 mil. If I try spraying thicker my defects just get magnified. I face the same general issues regardless of waterborne product, so it must be something fundamental that I'm missing.

[Jim Becker]

Have you tried a different gun? How about the larger 1.8mm setup on the current gun? Something is amiss (which you clearly know) when you are having the same issues with multiple products. I use a relatively inexpensive Wagner HPLV conversion gun and get pretty consistent results whether I'm spraying EM6000, de-waxed shellac or even Minwax Polycrylic.

[John TenEyck]

1-2 mils is not enough; no wonder it's not flowing out well. You need to lay down at least 3 mils of any of the GF products, TC's, too, IIRC.

Crystalac does not shrink when it dries, and you should not need to fill the grain more than 2X, once is usually enough, but 2 doesn't hurt. If you fill the grain and seal it with Sealcoat, etc., then apply 3 coats of Clear Poly, sanding with 400 or 600 between coats, there should be no reason for you to have to rub it out. It should be near perfection straight off the gun.

If you don't have a pressure feed gun you should use a 1.8 mm N/N with PreCat lacquer or Clear Poly on a gravity feed HVLP gun.

John

[Denis Kenzior]

John,

I've heard conflicting advice on the mil thickness. Many experienced users recommend thin coats, just enough to flow out. From my experience the defects just get amplified the thicker the coat. Particularly pin holes and bubbles take some sanding to get out. So I need to eliminate them first.

Any reason you're recommending a larger tip? General Finishes specs 1.1 to 1.3 mm tip for PreCat. I can use a pressurized gravity cup or a pressurized siphon setup, but never felt the tip size was too small.

I know you use a Qualspray gun, which is essentially the same as the Asturo above. What gun settings, air pressure, fluid pressure do you typically use? Do you thin? How many passes do you make, crosshatch, etc?Any chance you can post a picture of what a wet coat off the gun should look like? Do you spray horizontally or keep things upright?

I will try to prepare some test boards on beech next week to eliminate the need for pore filler. Would like to get to the bottom of this.

[John TenEyck]

It takes about 3 mils for the GF finishes I've used to flow out. Maybe 2 mils will, too, but 1 mil never will unless maybe the RH is super high or you add a lot of extender to it. GF recommends 3 - 5 mils for PreCat Lacquer.

I thought you were using a gravity feed HVLP gun. That's why I recommended a 1.8 mm N/N. That's what I used to use with my gravity feed gun. Clear Poly has a viscosity of about 45 seconds #4 Ford cup. A 1.8 mm N/N is in the right range for that viscosity with a gravity feed gun. GF's recommendations are based on a pressure assisted gun. They gloss over that in most of their literature.

When I spray Clear Poly with the Qualspray/PPS cup gun I use a 1.0 mm N/N. I set the air pressure at 29 psi, the cup pressure at 5 psi, the fluid knob a 1-1/4 turns open, and the fan at 1 turn open. I don't thin it. I overlap passes by 50%. That will give you about a 3 mil wet thickness depending upon your pace of course. I try to pace so that it looks like an almost but not quite a complete film when it's first sprayed. A couple of minutes later, however, it will have flowed out into a continuous film. I try to spray horizontally as much as possible but sometime you have to spray vertically. I'm extra careful then to avoid runs and sags, so maybe I'm spraying slightly thinner coats then.

John

[Wayne Lomman]

If your finish bubbles and the bubbles pop before it flashes off, that is normal. You apply a coat, sand it, and recoat. Repeat until the grain is full. Before the final coat you need to block it flat to get rid of the ripples/orange peel.

If you get bubbles that are still there when the coating is dry, you need to slow down the drying rate to allow the timber to outgas before the coating flashes off. This can be done by adding a retarder thinner or by adding more thinner so that air can escape more easily.

Practice your techniques on scrap. Change one variable at a time from test to test so that you know what effect your changes have. Cheers

[Denis Kenzior]

So I ran some more tests. I switched to a CA Technologies Lynx 100H gun (siphon fed). This gun operates at 18 psi inlet. Also added an extra pressure regulator for the fluid. Using a 1.2 N/N setup and 5 psi fluid pressure, I only need about half turn open to lay down a fairly heavy coat (though I still move somewhat slow compared to you experts). Using a siphon setup also allows me to warm the finish slightly on a hot plate (barely warm to the touch) prior to spraying.

Sprayed some prefinished maple flooring at 3 mil wet. The sample was wiped with a 50/50 DNA/water mix.
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Still getting pimples and fisheye. The finish dries wonderfully smooth, but one can still feel the dried pimples. What could be the source of contamination, just airborne dust that gets forced into the finish by the spray gun? From the compressor? (It was drained, dessicant replaced prior to shooting and filters seem clean). Or is this just dried bits of finish forming in the cup (strainer was used to fill the cup)?

[Wayne Lomman]

Denis, is this the result from spraying one coat? You don't say anything about sanding between coats. Cheers

[John TenEyck]

I know I said 3 to 5 mils, but that looks thicker than even 5 mils. If you don't have a set of jeweler's glasses you should get some. Less than $10 on Amazon. With 10X you will be able to see what the source of those pimples is. I had awful problems with a TC satin sheen product. The jeweler's glasses showed the source was bubbles that didn't pop, or partially popped but then dried. I sprayed light, I sprayed heavy, didn't matter. I switched products. FWIW, I've never had this problem with GF's Enduro Clear Poly. Of course, there's no saying your problem is due to bubbles, but that's my first guess.

Were you using the GF PreCat Lacquer?

John

[Denis Kenzior]

Wayne,

The latest sample I sprayed (in the picture above) was a pre-finished flooring plank left over from my floor install. It had a good thick layer of finish on it, completely flat. I wiped it down with Naptha and then 50/50 denatured alcohol water mix just to eliminate any contaminants it might have picked up. Was wanting to eliminate any other factor that I could be introducing, e.g. improper sanding between coats.

Incidentally I use 220-320 grit 3M 216U paper or Abranet for between coat leveling. Then vacuum then wipe with 50/50 DNA-water mix.

[Denis Kenzior]

John,

There are left over impressions from my wet mil gauge in the picture. There are 3 or 4 middle teeth impressions, which corresponds to 75 to 100 um (3-4 mil). I pulled it at a slight angle, but it shouldn't be a heavier coat than 4 mil or so.

I tend to spray lighter coats than that, just wet enough that it wets out and looks wet but not wet like it was applied by a brush. The surface tension of water makes it pool up at the edges and the thicker the coat the more noticeable this effect is.

I will get the Jeweler's glasses ordered. Thanks for the tip. If I had to guess, I think that the bubbles are happening here as well. I do get bits of random lint or small hairs trapped in the film, but these are few. Maybe one for 3-4 panels sprayed. What I'm seeing looks more like pimples and craters. The pimples tend to shrink as the finish continues to cure (e.g. over the next several days) and are much less noticeable by feel in 48 hours. The crater goes right through the coat, these are much harder to deal with and seem completely random.

Everything I've sprayed so far was with GF PreCat.

[John TenEyck]

I use the wet mil gage by simply putting it straight down into the finish. No angle, no dragging. Down, up, inspect. Right or wrong, that's how I gage the wet mil thickness. The tallest "leg" of the gage with finish on it is the wet mil thickness.

John

[Jim Becker]

Just to provide some solace...I'm have some real struggles with finishing a project "as we speak", too...so many "moving parts" and while I'm inching closer, I'm not were I need to be to get the glass-smooth surface I'm aiming at. Perhaps it's just because I'm using a different product than previously. Some of it was not paying attention to the teeny-tiny type on the gun components and getting a mis-matched setup for the first few coats. At least it's going to be a darn thick finish that will be pretty durable for the purpose. LOL

[John TenEyck]

I'm sure you're not alone, Jim. I'll bet a poll of fellow Creekers would show at least one finishing disaster in the last few months. Count me among them. It was a new product for me, too, one I won't be using again.

John

[Jim Becker]

I'm sure you're not alone, Jim. I'll bet a poll of fellow Creekers would show at least one finishing disaster in the last few months. Count me among them. It was a new product for me, too, one I won't be using again.

Yea, I'm disappointed since it would have been nice to patronize the local SW store. At least I have a really good, thick, transparent, albeit wavy coating on the thing at present which I'm going to level and then coat with a Target Coatings product which I know sprays well with my gear. Sheesh...