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How much to sand for filler

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n7188u

Joined: 15 Nov 2015
Posts: 404

Posted: Sat Oct 03, 2020 9:09 am Post subject: How much to sand for filler

Dear forum members,



I always had a question, never having a chance to see in person a properly done sample of how much you should sand a control surface layup before applying filler to the control surfaces. 



If you lightly scuff the surface with 80 grit all you get is minute dullness at the crest of the layup fiber bundles. I would say not even 10-20% scuffed. If you go more then you are indeed cutting through the fibers. but my understanding is that anything that is still glossy provides basically zero bonding strength. 



So what is that magical point at which you stop sanding and it is sufficient bonding for the filer.



BTW, one time I spoke with a gentleman that happened to run the R&D department at Lancair during the certification of the Columbia 300. He told me they did extensive testing on how much to sand before a structural bond. He indicated that the highest structural strength with bonded components would be achieved by doing very detailed cleaning and then just using scuff pads to lightly scuff the surface. The resulting bonded structure was stronger than cleaning and sanding with coarse sandpaper until all surfaces are dull. Now, he explained we are not talking about the bond strength. He explained that what they tested is for the overall strength of the bonded structure and his explanation is that using coarse sandpaper damages the substrate layups and overall you end up with a weaker structure. Then he added they still ended up scuffing until dull in production because the FAA would not accept that just using scuff pads and ending with a surface that still looks glossy would be OK despite all the testing.



I found the story interesting.



Best regards,

Chris

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n7188u

Joined: 15 Nov 2015
Posts: 404

Posted: Sat Oct 03, 2020 9:24 am Post subject: Re: How much to sand for filler

BTW, Rutan wrote in his booklet "Moldless Composite Homebuilt Sandwich Aircraft Construction" that if you wait until the epoxy fully cures "you will have to sand the fiberglass surface to a dull finish". This is for application of contour fill.



Hence a lifetime of confusion for me.



When we discuss this can we estimate how much dullness you want in terms of % of dull surface?



Best,

Chris

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ward.t(at)xtra.co.nz
Guest

Posted: Sat Oct 03, 2020 11:55 am Post subject: How much to sand for filler

Hi Chris,

My Air New Zealand painter who painted my aircraft Boeing White, said to go down to 180 grit only. Finer will cause a lack of bonding for the paint.

Tim



Sent from my iPad

Tim Ward

12 Waiwetu Street, Fendalton, CHRISTCHURCH. NZ 8052 

ward.t(at)xtra.co.nz 

021 0640221



 	  Quote: 	
		   On 4/10/2020, at 6:21 AM, n7188u <chmgarb(at)gmail.com> wrote:

 

 

 

 Dear forum members,

 

 I always had a question, never having a chance to see in person a properly done sample of how much you should sand a control surface layup before applying filler to the control surfaces. 

 

 If you lightly scuff the surface with 80 grit all you get is minute dullness at the crest of the layup fiber bundles. I would say not even 10-20% scuffed. If you go more then you are indeed cutting through the fibers. but my understanding is that anything that is still glossy provides basically zero bonding strength. 

 

 So what is that magical point at which you stop sanding and it is sufficient bonding for the filer.

 

 BTW, one time I spoke with a gentleman that happened to run the R&D department at Lancair during the certification of the Columbia 300. He told me they did extensive testing on how much to sand before a structural bond. He indicated that the highest structural strength with bonded components would be achieved by doing very detailed cleaning and then just using scuff pads to lightly scuff the surface. The resulting bonded structure was stronger than cleaning and sanding with coarse sandpaper until all surfaces are dull. Now, he explained we are not talking about the bond strength. He explained that what they tested is for the overall strength of the bonded structure and his explanation is that using coarse sandpaper damages the substrate layups and overall you end up with a weaker structure. Then he added they still ended up scuffing until dull in production because the FAA would not accept that just using scuff pads and ending with a surface that still looks glossy would be !

 OK despite all the testing.

 

 I found the story interesting.

 

 Best regards,

 Chris

 

 

 

 

 Read this topic online here:

 

 http://forums.matronics.com/viewtopic.php?p=498640#498640

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budyerly@msn.com

Joined: 05 Oct 2019
Posts: 286
Location: Florida USA

Posted: Sun Oct 04, 2020 1:14 pm Post subject: Re: How much to sand for filler

Excellent discussion.  Either building new or fabricating a new do-dad for the plane I do the following:



After layup, I fill the surface with microballoons to fill the gaps.

This leaves a filled weave (sort of) that can be easily scuffed for filler.  Clean and wipe with acetone to get your dust and paw prints off.  



If not, I use 80-150 grit for a light scratch then use red or green Scotch-Brite to rough up the dimples in the surface to a sort of dull state.  Clean.



If using expand cell as a filler, I use a slightly wet fill then even out to a very thin layer.  The extra epoxy is heavy but it sticks for sure.  Allow to cure and sand with 80-150 grit and my DA sander to get back to near the glass.  Refill one more time with a drier filler.  The vibration of the pneumatic DA is higher than a hand sand, but it is faster and will tell you if you have an area that is not sticking well as you feel it and see the dry or not attached area to crumble away.



At Scaled Composites in the dry heat of the dessert, once cleaned up, one worker would lay on a thin clear epoxy coat and allow it to tack, then put on their filler which was normally micro and epoxy as a paste (witch is really itchy and irritating when sanding) but I used Poly-Fiber SuperFil which is expensive, but nice to use and easy on the lungs and eyes.  The reason the worker used for the clear epoxy was assured bonding in a dusty environment of the micro and the sticky epoxy allowed him to use a drier filler and get a smooth pull of the filler to a consistent, flat spread.  



Ideas are like butt holes, everybody's got one.  That's mine.



I did an article some time ago on finishing and painting, it is on my website https://www.customflightcreations.com.  



Bud Yerly

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