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Thread: Spray Finish Help - Please help me understand

  1. #1
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    Spray Finish Help - Please help me understand

    I'm working on the Spool Cabinet and intend to prefinish the individual pieces prior to assembly. Since I'm new to spray finishing, I'm very confused as to method. Hubby owns the HVLP guns etc as he sprays motorcycle components regularly, so actual application isn't a problem.

    1) Many of the pieces will require finish on all 8 sides. How do I do that? Do they have to be hung or can they lay flat some how and do one side and then another?

    2) How many light spray coats are needed to equal 2 brushed coats in thickness?

    Thanks for all your help.


    PS I expect to be using General Finishes High Performance Top Coat.

  2. #2
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    To your first question, there are many ways to spray all sides. Hanging is sometimes the answer, but can also be hard to control when the spray action moves the workpiece...you don't want that in any way, shape or form because it will cause you to get uneven results. Spraying flat can help with that. I will try to spray the "less important" side first, including the edges and once that's dried enough, I"ll flip the piece over to do the top. The reason I do the edges in the first step is to avoid as much as possible any "bounce" onto a finished surface from whatever is supporting the workpiece.

    If you're brushing on two coats first, the only benefit that spraying will bring is a little more build and, perhaps, a more refined surface. Why not spray all your coats? The finish you've chosen is ideal for spraying.

    BTW, given you are new to spraying you need to "Burn" some finish practicing the steady technique as well as to insure you have your gun setup properly for the finish you are using. GFHP isn't anything like what your husband likely sprays on those motorcycle components when it comes to viscosity and other factors, so you need to prepare for that. You may even need a different needle/nozzle setup for your finish compared to what he uses.
    Last edited by Jim Becker; 04-07-2021 at 9:37 AM.
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  3. #3
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    Thanks Jim. The recommended tip size is 1.1 to 1.3, I have 1.1, 1.2 and 1.3 so I should be okay there. We'll plan on plenty of samples.

    I'm not wanting to brush any of the coats, but rather just trying to figure out how many spray coats are the equivalent to 2 brushed coats since it is my impression spray coats are thinner. I'm planning on rubbing out the finish after it cures.

    Since the parts have ends that won't be visible, I think I'll find a way to mount them vertically.between pins or something.

  4. #4
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    Sprayed coats can and usually are thinner than brushed coats...because they can be. I normally spray about three coats of the "finish finish", meaning that's independent of any initial steps I might take, such as a light coat of wax-free shellac over oil and/or dye used for coloration. Sometimes I'll go to four coats. In the end, it also depends upon your skills which is why I suggested you need to burn some finish practicing. Spraying isn't "hard to do", but you have to get comfortable with the motion and comfortable with laying down an even coat that's not too sparse or not too heavy. The latter can be worse than the former, not just because of runs on vertical surfaces, but too thick can lead to other issues such as excessive "orange peel", etc.
    --

    The most expensive tool is the one you buy "cheaply" and often...

  5. #5
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    Quote Originally Posted by Lisa Starr View Post
    Thanks Jim. The recommended tip size is 1.1 to 1.3, I have 1.1, 1.2 and 1.3 so I should be okay there. We'll plan on plenty of samples.

    I'm not wanting to brush any of the coats, but rather just trying to figure out how many spray coats are the equivalent to 2 brushed coats since it is my impression spray coats are thinner. I'm planning on rubbing out the finish after it cures.

    Since the parts have ends that won't be visible, I think I'll find a way to mount them vertically.between pins or something.
    TDS for GFHP is here :
    https://generalfinishes.com/sites/de...l-Finishes.pdf

    Recommends a maximum of 5mil total dry finish with a wet application between 3-5mil (mil = 0.001"). You can get a mil gauge at a decent paint store and use it as part of your practice. Probably even try brushing or rolling a bit to see what the difference is. I generally assume the dry mill thickness to be about 1/2 my applied wet mill thickness. I'm probably over estimating but close enough. So in the absence of other information on the TDS, two sprayed coats of 3-5mil wet is right about on the max for dry thickness.
    Don't sweat the petty things and don't pet the sweaty things.

  6. #6
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    If your gun is a gravity feed gun none of those needles is large enough w/o substantial thinning. High Performance has a viscosity of around 65 seconds #4 Ford cup in the can. I used to thin it 6% to get it down to around 45 to 50 seconds which is what you want for a 1.8 mm needle. With a 1.3 mm needle being your largest you will have to thin it more, probably at least 10%, but get a #4 Ford cup if you don't already have one to test. If your gun is a pressure assisted gun then use the 1.3 mm needle w/o thinning.

    There's no good reason to rub out High Performance and plenty of reasons not to. Spray one or two thin coats, lightly sanding with 400 grit in between, and then spray your final coat. It should be good enough off the gun for anything short of a piano finish. If you do decide to run it out then wait about a week for it to fully cure before doing so.

    I had a real chuckle when you said the pieces required finish on all 8 sides.

    John

  7. #7
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    John, Thanks for the additional info. I just checked the supply of guns available to me, and we do have a gravity feed gun with 1.8 and 2.0 tips. Sounds like your experience of thinning for the 1.8 tip should do it for me. I'll continue more research on that.

    Sorry about the 8 sides...that's what happens when you have finished the 1st cup of coffee
    Last edited by Lisa Starr; 04-07-2021 at 2:54 PM.

  8. #8
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    Lisa there is good info for that finish on Generals web site. You will want to look for the TDS Technical data Sheet. One thing you are after is the Volume Solids content % .. the High Performance Top Coat has a Volume Solids of 25% which means if you apply 5 wet Mils after it dries the Dry Film Thickness is 1.25 mil ( 5 wet mil X .25 VS = 1.25 DFT ) ... they recommend a finished dry film thickness of between 2.5-5 mils .. so applying 5 wet mils you will need 3 coats to achieve there recommended final coating thickness with a light sanding in between coats. In addition adding any reduction will also lower the VS % of the product ..

  9. #9
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    I just checked GF's TDS on HP and see that they continue to recommend a 1.1 - 1.3 mm needle. What they don't tell you is their recommendations are based on using a pressure assisted spray gun. If you dig deeper in the FAQ's you'll find this video where they spray HP and talk about the guns they use. Both have a 1.5 mm needle. The one in the demo is an Earlex, which is a pressure assisted gun. No wonder customers get confused.

    https://generalfinishes.com/videos/h...based-top-coat

    John

  10. #10
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    Thank you Robyn. Another piece of the puzzle. Due to my husband's motorcycle habit, I have several cups, the comb gauge and multiple sprayers available to me for me to learn with. I'm planning on burning thru a lot of material getting this right, but it should up my finishing game once I make it thru the learning curve. I enjoy the "how and why" of processes, so I appreciate you bringing the Volume Solids calculation to my attention.

  11. #11
    I'm also new to spraying, just finished my second major project with my HVLP. The good news is it's not actually that hard, and good finishes, at least, are very forgiving. The bad news is that between needle size, fluid knob, fan width, air flow, and viscosity you have 5 knobs you can turn that are inter-related, so e.g. if you change fan width you also need to change fluid volume to match. I would definitely "burn" not only finish but some less valuable projects before you do the first project you really care about/have already invested a hundred hours. The hardest part for me has been not getting too much in areas where passes overlap (e.g. doing the leg of a table and then the rail)

    My approach has been to limit the variables my only spraying water-based (target in my case), using the recommended needle and full air. Then that leaves me to adjusting the fan width and fluid level. I practice on a piece of scrap to make sure I'm getting about the right thickness, then spray the piece.

    The other thing I've learned is despite it seeming intimidating, setup and cleanup is relatively easy and quick. It's much easier to let things dry and come back for more passes later, even if it's the next day.

    You said you're planning on prefinish prior to assembly. I've done that on both of the big projects I've done. Make sure you tape etc the glue surfaces. I have a set of spare dominoes I have used to plug those holes...

    Bruce

  12. #12
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    That's a good strategy, Bruce. I've added a couple more things that to reduce confusion. I leave the on gun air valve wide open; never touch it. Air volume is adjusted by setting the pressure regulator, and most of the time that stays at 29 psi. Similarly, I leave the fan at full width 95% of the time. Too much overspray for narrow parts? Not really; just rotate the gun so the fan is on a diagonal, or forget about it as I do most of the time. I don't really care if I waste a few ounces of finish. The point is, spraying is pretty easy if you keep it simple.

    John

  13. #13
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    Thanks to everyone's help, I successfully completed my sample finishes for the spool cabinet today. I ended up using GF High Performance Top coat over Oil Based Dye. The dye was mixed with Lacquer Thinner before applying and then allowed to dry 9 days. My spray gun setup ended up being the Gravity Feed Gun with a 2.0 tip and the finish thinned 3%. This allowed me to hit a 4 mil wet finish, which should dry to approximately a 1 mil dry thickness. After 3 coats today I'm very pleased with the look and feel of the samples.

    For the actual project, I expect to apply 2 coats prior to assembly and then 2 or 3 more coats after assembly. This should put me right at the 3-5 mil dry finish thickness.

    Again, thank you everyone for your help

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