Pre-treating paulownia or balsa core

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vinman
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Pre-treating paulownia or balsa core

Post by vinman » Tue Apr 30, 2019 5:09 pm

I'm getting ready to build my first paulownia core skis. I've read and been told that Paulowinai soaks up a bunch of resin. I've also read that certain builders "pre-soak" their light weight cores in resin. I'm assuming to prevent excessive resin uptake during pressing.

Does anyone have insight to this idea?

I was thinking of a light brushing with epoxy and allow it to cure prior to lay up as a way to limit the amount of resin uptake during pressing. Brush on only enough to close the pores but not leave a glossy surface...enough open pores to still create a solid bond.....?

Ideas?
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Re: Pre-treating paulownia or balsa core

Post by chrismp » Wed May 01, 2019 1:58 am

That might work. I would try to use a resin that has pretty high viscosity, so it doesn't seep into the pores as easily. Probably a good idea to do this right before pressing and start pressing once the resin started to gel and gets tacky. That way the epoxy for the reinforcement layers has a good bonding surface on the core.

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Re: Pre-treating paulownia or balsa core

Post by vinman » Wed May 01, 2019 3:42 am

I was thinking thinner resin so it soaks in but does not totally seal the top surface but just kind of creates a less porous surface so the thicker laminating resin does not get soaked up too much....

On Wagner’s web site they mention soaking their lightweight wood cores in resin. This is where I got the idea.
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Re: Pre-treating paulownia or balsa core

Post by MontuckyMadman » Wed May 01, 2019 9:22 am

I know u think I'm crazy but a super thin water based poly topcoat would seal and be porous to accept epoxy and good for bonding in all my tests. Cheap, quick and easy.
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Re: Pre-treating paulownia or balsa core

Post by vinman » Wed May 01, 2019 10:50 am

MM, I’ll have to test that I guess.

I may just lay up as is just to see what happens. My buddy in big time snowboard biz says it isn’t really needed...

I’ll see I guess.
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Re: Pre-treating paulownia or balsa core

Post by skidesmond » Sat May 04, 2019 6:18 pm

Sounds like it’s going to soak in resin either way, pre-soak or not. No idea if this would work but it’s the same idea.... I read in a woodworking mag that if you want to get an even stain over an open grain board like oak, etc. to apply a light coat of wood glue on the end of the board. The end grain will soak up the glue. Then lightly sand, then apply stain. This is suppose the give the end grain of the board and the face of the board the same stained look. So do you think wood glue would help seal the pores?

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Re: Pre-treating paulownia or balsa core

Post by vinman » Mon May 06, 2019 7:52 am

I was thinking watered down wood glue would work well. I may try to test this on some scrap this week.
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Re: Pre-treating paulownia or balsa core

Post by 24Dave » Mon May 06, 2019 6:36 pm

I've seen average viscosity resin move through 8-10mm of paulownia when cured at 140 F. End grain balsa is commonly sealed in commercial applications, I've never used it. I have also sealed cores to keep them light. Nothing technical to it. Use a squigee, spread resin across part of your core then scrape as much off as you can with pretty hard pressure fairly quickly. After it cures, I give the core a quick light sand with some 100 grit in case there is blush from the epoxy that might make for a weaker bond. After having made several boards with paulownia, I generally think epoxy in through the pores of this wood is generally a good idea-especially on the top side to increase compression resistance.

One thing this process does though if you are using really thin laminates is guarantee that your fiber won't starve fro resin that soaked into the core, I could see this happening with Textreme or stacked uni layers that you have already put resin in and scraped fairly dry on a separate table for weight savings. I doubt it's an issue with triax fiberglass because there is so much room for resin in the weave.

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Re: Pre-treating paulownia or balsa core

Post by gav wa » Tue May 07, 2019 1:04 am

As Dave has mentioned just give it a coat, then light sand before layup. I have simply brushed resin on before. It will penetrate much less than it does when heated and under pressure of the layup so will work as you need. You need to do both sides at the same time though and cure on its side or it will pull a bow in your core, end grain balsa pulls real hard.

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Re: Pre-treating paulownia or balsa core

Post by vinman » Tue May 07, 2019 6:00 am

Great info!! I think this is what I will try. I’m guessing that it really doesn’t need much resin at all to partially seal the pores.

Thank you!
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Re: Pre-treating paulownia or balsa core

Post by mammuth » Wed May 08, 2019 8:31 am

Roll it on. You can get a light coating without the mess and material you need with scraping
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Re: Pre-treating paulownia or balsa core

Post by vinman » Wed May 08, 2019 8:51 am

For this core I only have 2 19mm strips in the core. I’ll paint it on just these stringers and scrape down the excess, give them a light sanding and lay them up.

I’ll probably tape the stringers next to them to keep things from getting messy also.
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Re: Pre-treating paulownia or balsa core

Post by vinman » Tue May 14, 2019 2:25 pm

So I did pre-treat these stingers with some thin epoxy after taping out the other areas of the core that did not need to be treated. It seemed to soak in well.

I applied a trickle of resin and scarped it in as well as wiped off any excess with a paper towel. Today I did a light sanding of the treated areas.

They do seem a bit harder to a fingernail dent test so I think it did take up some resin and is now partially sealed which seems like about as much as was hoping for.

I'll be laying these core up in about a week and a half. I have a the same pair with my traditional core in them to compare to with the same core. So we will see what happens. I'm hoping for .5-.75 lbs decrease in each ski while maintaining performance.

The auto road on Mt. Washington opens around Memorial Day so I'm hoping to be able to test these once that opens. There will be skiable snow there until mid-June.
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Re: Pre-treating paulownia or balsa core

Post by vinman » Thu May 30, 2019 4:29 pm

I finally got to press these skis today. The finished ski with the paulownia core is ~9 oz lighter than the same ski with my traditional core.

The ski is 182cm 135/108/123. The old cores ski weighs ~4.75 lbs per ski, the new core skis weigh 4lbs 3 oz per sk

I used only 2 19mm strips of paulownia per core and replaced 2 strips of maple.

The flex on this new set seems great with a medium/snappy feel and very similar to the original.
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Re: Pre-treating paulownia or balsa core

Post by gav wa » Sat Jun 01, 2019 4:50 am

Sounds pretty much exactly what you were hoping for yeah?
Nice work.
It is amazing how sensitive you get to different finished weights after building a few hey. I pick up two boards with 200 grams difference and they feel worlds apart.

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