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Sharing here some findings while testing film thickness.

Background: I spray UV cured polyester from Cardinal. Typical schedule that I think most people use is to spray two overlapping coats, cure, and then spray two more.

What this results in is a 2-3 mil base layer and a 2-3 mil top layer totaling 5-6 mils of finish before leveling & buffing. This depends a little on the brand of material and if using any reducers. In my case I’m using Cardinal U4PA-CLE22 polyester which is 65% solids.

Using the above two coat approach also requires sanding between coats at 220 / maroon scotchbrite. This is critical because the material is all mechanical bond and failing to properly tooth the surface between coats can lead to adhesion failure (ex: Taylor horror stories).

This also creates a concern about going through the top layer and having witness lines from the 220 scratches between coats. How to solve this?

My thought has been to wet stack three coats rather than “two - cure - two.”. The obvious concerns are:
- Getting it to hang properly with no runs or sags
- Spraying three “lighter” coats may result in excessive orange peel.
- Curing the thicker material properly

Using a sprayout card I sprayed three wet stacked coats of around 4 wet mils which created a dry film of around 6.8-7.4 mils (measured in various places on the card). Note that the 1.4 mil second layer is the sealant on the card.

The images below are measurements of a specific area at each step of leveling and buffing. You might ask why I would go from 400 grit paper backward to 360 grit Assilex. I’ve found that the 600 grit Assilex on a ROS cannot get hand sanded 400 grit Cubitron scratches out.

Was quite surprised too by the amount of film removed by the medium cut Menzerna. This was all focused on a small area so the buffer may have removed more than typical.

Any feedback / recommendations / etc… appreciated!













Brad


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I’m jealous of your device….

Back in my wet sanding days, we’d level with 800, switch to 1500, then go to the wheel…although I think those numbers are equivalent, we actually used 15 and 30 micron…

I’m definitely getting there. It doesn’t make sense to chase the rainbow with the Eagle sanding discs. It’s nice to look at but mucho dinero.

I definitely cannot level with 800 for two reasons, I don’t have the spray technique to do it and the polyester is tough as nails. Getting there. For now I have to level at 400 and go up.

I’m in agreement about the upper bound too. Sanding to 3K and then going to medium cut Menzerna doesn’t quite make sense. I’m still working out the right mix of all of it.

For now I just stay methodical and over time I’ll likely drop things out of the mix.



That Mirka sander is banger though!


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Interest posts.

Finishing remains my most difficult step.

Definitely one of the hardest steps. I stick with nitro as it's what I know and am set up for. It's still easily available here.

I think you’re right though about having only one layer. It makes no sense to me to have a cured base layer and then a second layer that you hope you don’t go through. That only encourages a thicker finish.

Is the sealer necessary on spruce? Cause if you could not have the sealer and get a 3.5 topcoat that’d be pretty good. Having that meter would tempt me to go for a .002 finish. Since you have the ability to measure and be certain, you could probably achieve that.

Once you get your instruments up to a certain level of responsiveness, finish can make or break a guitar…

I’ve done it both ways on spruce, with and without sealer, both with Simtec and Cardinal. Those instruments are fine, but Cardinal definitely recommends sealing.

Yeah, the Positector is pretty much crucial for this to work because of the initial thickness of the one coat. I feel like I can basically take it down pretty low on the top, maybe just waist and down.

We will know soon enough, I’m spraying a body tomorrow!


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I wish I could afford the Positector, but, I make guitars for a living, lol…

Do they specifically recommend sealing spruce, or just sealing in general?

TDS - http://cardinalpaint.com/assets/Uploads ... -J11TL.pdf

The TDS doesn’t call for it specifically but the tech at Cardinal said yes, isolante on all woods is required.


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Is it possible that’s just CYA speak?

Could be. I did some testing a while back and sealed wood scuffed with maroon scotchbrite performed the same as bare wood sanded to 220 - viewtopic.php?t=55617


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If it were me I would definitely be leaving the 1.5 mil out if I could. Bindings and rosette could be carefully sealed with brushed shellac…