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Pinholes - Waxed Polyester - Help Needed12/27
The product I have been using is the Sayerlack PFL6269, and I had similar results with the Sherwin William PL4234.
∑ Mix Polyester with 2-4% Cobalt and 2-6% catalyst, depending on if it is the fast or medium catalyst, I use up to 10% acetone to thin. I have played with different mixes though get the same troubles. I know these mixes aren't standard for polyester, I have been following the Sayerlack datasheet.
∑ Gun settings. 1.8mm tip, 20-22psi, fan wide open and 3.5 turns on the fluid control. I have recently experimented with winding the fluid nozzle in a bit and making the fan a little narrower (about a hand-span wide) though I still get the same results.
I apply the first coat of polyester. It is important to note that I often get tiny air bubbles/pinholes as soon as the first coat is sprayed. In recent experiments, this hasn't happened since winding in the fluid control and spraying with a narrower spray fan.
I apply the second coat as the first coat starts to gel (anywhere from 3-9mins depending on the weather) and repeat for the 3rd, 4th and 5th coats sticking to the interval as determined from the first coat. My second, third, fourth and fifth coats alway lay down nice and clean with no sign of pinholes or bubbles.
The pinholes show up when the coating starts drying. When I sand the coating flat before buffing (normally 240g-1500g) the pinholes cannot be seen but once the panel is buffed the pinholes show on certain angles. They are tiny.
Please note; before mixing up a batch of polyester, I clean the mixing cups with acetone and rinse the spray gun with acetone before spraying.
My airline runs through a refrigerated air dry at the spray booth than through another filter at the booth.
The first photo is the polyester wet after the 3rd coat and the second photo is the polyester drying after the 4th coat with the pin holes coming out.
How heavy are you applying? If too heavy, could be airbubbles that werent able to escape
Itís possible that the coats are a bit heavy. I need to get a thickness gauge and play with that. I am applying full wet coats but Iím not dumping it on. All of my work is sprayed vertically so I canít apply too heavy or it will just run.
I am planning on doing a few more samples next week with thinner coats to see if I get a different result.
I have never used the sayerlack/sw polyesters, though I have used other brands through the years. My initial thoughts are in the area of solvent pop - ie I would look at the acetone you are using. Also, there are a number of polyesters available that allow for a much lower percentage of cobalt in your batch formula. This allows for a greater open window for the cross linking/ gelling process. for instance, rather than the 2% cobalt add, one can use .5% ( let's say ) which then creates a longer open time which then allows for a greater opportunity for the coating to level/flow out.
Good luck with the fires. May you get rain.
Shenendoah, thanks very much for your input.
I actually thought the acetone may of been part of the issue so I have also tried a slow polyester thinner from SW though I got the same results. The acetone I use was recommend and is from SW. It might be worth getting a different acetone and seeing if it helps.
Unfortunately with the current drum of Sayerlack Polyester the supplier pre mixed in the cobalt when shipping it out so I haven't been able to adjust the cobalt on this batch. On the last batch of SW Polyester I had control of the cobalt and tried mixing in less cobalt but has a similar result as well.
Regarding the isolator, It is a SW product (TL4570), a 1:1 mix with no thinning required. I often scuff sand with 320g before applying and polyester.
On the next samples I will thin the polyester more as an experiment and see if it helps.
Re air flow. I was hiring different booths for a while and recently had a new booth installed in to my workshop. With the new spray booth we had the fan geared down so it's not moving the air too quickly, one thing we wanted to make sure that the paint has enough time to settle on the substrate before the over-spray is evacuated. I have had this issue in all the booths I have used.
We definitely need the rain, the conditions are pretty bad at the moment.
Steve, Spray your PE mixture on glass or laminate just as if you are finishing wood.
Since your PE is parrafinated are you waiting to recoat when your PE gets to the "string" stage. This is when you touch it and it will pull strings.
Main thing is look at it under magnification we need to know does the bubble go all the way down to wood.
Thanks for the advice, Bob, it's greatly appreciated.
I actually have done a sample on a piece of glass to see eliminate that the issue was coming from the veneer substrate. I will see if I can dig up a photo. My results were the same when I sprayed the glass. I will try again next week.
At first I was struggling to get the gel time right though now I'm more comfortable In your experience would you let it set up a bit more?
I haven't had a chance to get the coating under a magnifying glass. I will try get my hands on one and have a look. I do get the pin holes even if I spray a surface that is already grain filled and sealed with either polyester or 2k urethane.
I use a Star Evo T 1.8mm gun, I believe it's a LVLP, do you think this is suitable or would you recommend a different gun? could the pin holes be from applying coats that are too heavy?
Since you had the same issue on glass it is your Polyester mixture. Most PE will work fine @ 4-6 mills wet per coat. I looked up the TDS on your products to better understand the product.
When I sprayed on glass I only sprayed one coat, and straight away it looked a little bubbly in areas, Photo attached. Note, This generally happens on my first coat over veneer substrate or a sealed substrate, the first coat always seems a bit funny, but the subsequential coats go down nicely. I did a sample on a veneer substrate the other day and it didn't seem as bad after I wound in the fan and fluid control a little bit. I think I also need to try another gun.
Re the mixture, based on the information provided by my supplier, I have been mixing by volume. I could very well be wrong here though an example of my mix would be, based off a mix with 4% cobalt, 2% catalyst and 10% acetone the mix would be.
Correct, the pinholes show when the coating is drying. When the coats have been built up and wet the coating looks very clean. The pin holes don't show between coats, they come out when I have finished a session and the piece is drying, I tend to notice it when the wax starts coming to the surface. When the coating is fully dry I can't see them with my eye. I can sand the panel flat and it looks fine, once buffed they are obvious. I have attached a photo of a buffed finish showing the pinholes (at it's worst).
I hope this makes sense. I don't have any issue with other coating, lacquers, 2k urethane, acid cat lacquer etc.
Most PU/PE formulations are given as percentages by weight, not volume. PE has a density of around 8.5 lbs/gal and acetone has a density of about 6.5 lbs/gal. This would mean that you are under reducing the mix.
How much airflow is going past the product while it's curing?
I cannot speak to polyester finishes but I've worked in booths before that had a lot more CFM than I'd care for blowing by my freshly sprayed product.
The result, even in cool weather, was a type of solvent pop that looked a lot like this. It was on solvent based lacquer and conversion varnishes (we tried several).
The airflow would skin over the surface faster than the underlying solvent could do anything to keep the film open, and you'd get pinholes.
Steve, what is your temp in the booth?
Thanks for the heads up. I will keep this in mind and run some experiments when I'm back in the workshop. Hopefully it helps.
Thanks for commenting. I have considered this and do need to run a couple of experiments as generally I leave the jobs in the booth with the ventilation on while they dry. I'll spray some panels and leave them out of the booth between coats and while drying.
I've haven't had any issues with other types of coatings while leaving them in the booth between coats and while drying. While I cannot tell you exactly what the airflow is in the booth, it's definitely only subtle and you cannot feel the air moving while the fan is running.
I have an open face spray booth so temperature in the booth varies with the weather and ambient room temp.