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Being a big fan of KTM-9, I decided I would give their SV a try. I was impressed with the material, sprayed really well, less likely to have sags than KTM 9 and no blue haze!

I sprayed 12 medium coats (6 one day, six the next, light sanding between days) and let it set up for about 10 days. It sanded quite well and it was ready for the buffer in no time. It buffed up well and I was ready for final assembly, or so I thought.

Seems when you turned it just the right way, in the right light, you could see a squiggly pattern under the finish. It looked like the trapped water you see under a tinting film put on glass using water. But in my case it didn’t just evaporate away.

My lay up for this guitar: Zpoxy on bare wood, level sanded followed by two light coats of wax free spray shellac. Light level sanding again and then the SV.

Using the shellac is also knew to me. I think the problem came from sanding the shellac to the point where it wasn’t a complete coat.

Anyone have any experiences like this?

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Joe Beaver
Maker of Sawdust

I think you hit it... Could be from sanding thru the shellac....

I had some really weird experiences when using shellac for an undercoat....

Sure - most things stick to it... but most things *DON'T* stick to it as well as they stick to bare wood.... Then - there's a slight difference in the way the shellac pops the underlying grain vs other finishes... so if you have places where there's no shellac - then places where there's shellac... you have places where the top coat sticks differently than it does on other places.... and places where the topcoat pops the grain 1 way, and places where the shellac popped the grain some other way...

Thanks

John, I think you may be right. It was a pain to fix

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Joe Beaver
Maker of Sawdust

Was it a rattle can of shellac that you used, Joe? If it was, I'm pretty sure the Zinser type shouldn't be used under a finish.

Alex

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"Indecision is the key to flexibility" .... Bumper sticker

Alex,

It was the zinser. Weird looking pattern deep under the finish.

Live and learn

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Joe Beaver
Maker of Sawdust

I have been using target coatings em 1000 under the em 6000 instead of shellac....
em 1000 is clear and great for dyes,

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MK5acoustics.com

With KTM 9 I just spray it on bare wood on the soundboard and over z-poxy on the rest of the guitar.

On this one to fix it, I sanded the bad areas down to at least z-poxy and some places to the wood. Applied more z-pozy and then sprayed with SV. No sealer coat. So far so good.

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Joe Beaver
Maker of Sawdust

Joe, it's not strictly relevant to the original post, but there's an old thread here: viewtopic.php?f=10101&t=29797 that discusses adhesion problems with KTM-SV when used over shellac seal coats.

I've used KTM-SV on my last three guitars, and have been pretty happy with it since the adhesion issues were solved.

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Cheers
Pete

I use shellac seal coats frequently. If you sand through anywhere, and don't sand it all off, you can get "witness lines".
I've left the seal cost intact and also sanded them all off, it's just a slightly different effect, and I think "better" may depend on the piece of wood.
I think the correct term is refractive index, that's what causes the grain to pop differently.

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A man hears what he wants to hear, and disreguards the rest. Paul Simon

Typically, shellac is not a great sealer for use under water base finishes due to the fact that shellac has poor resistance to water.

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Brian

You never know what you are capable of until you actually try.

https://www.howardguitarsdelaware.com/

I take from this that shellac was a bad idea. I'm hoping that SV over epoxy remains stable.

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Joe Beaver
Maker of Sawdust

Pete,

What was your solution to resolve the bonding issues?

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Joe Beaver
Maker of Sawdust

Joe, I managed to find a supplier of the two-part Ilva sealer Randy Muth recommended in the thread I referenced. Randy provided me with the following information and, although I wasn't able to find the exact products locally, I did find a very similar Ilva barrier coat product and a compatible accelerator.

"I get this stuff from Camger in Massachusetts. It's called ILVA Universal Polyurethane Barrier Coat. The overall product code is TF-25. Each component also has codes as follows:

Polyurethane Barrier Coat: TF-25
Accelerator: TV-19
Thinner: TZ-35"

After reviewing that old thread, I note that Rolfe Gerhardt reports no problems using Zinsser Sealcoat as a sealer and tie coat under KTM-SV. I couldn't find a supplier in Australia and went for the Ilva product instead.

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Cheers
Pete

Thank you Pete, I'll make a note of it.

One last question, you spray it on bare wood and over epoxy pore filler?

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Joe Beaver
Maker of Sawdust

For the back, sides and neck, I apply it over epoxy. For the soundboard, it's applied over bare wood. I've sprayed it so far, but I believe a foam brush does as good a job, with the added advantage that you're not filling the air with the stuff. Any product with the word "isocyanate" in the product description makes me nervous; I'd definitely use the Zinsser Sealcoat if I could find a source for it.

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Cheers
Pete

Thanks again. I think I'll give it a try

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Joe Beaver
Maker of Sawdust