Sanding Wood,Varnishing of Wood and Glue, When, Where and How?

[Sparrow]

Here is what I have been able to learn so far:
1.      I have concluded the varnish to use is the two part varnish that doesn’t have to be sanded off when putting on the wing and fuse covering.  Leonard said, in one of his posts on Yahoo, that is the way to go.
2.      The glue of choice is the structural adhesive T88. Use it sparingly on both surfaces. Hold the surfaces together until the epoxy cures weather it be staples, clamps, bands, straps or weights.  Wipe off any excess epoxy before it cures.

I have searched the posts I can get to and I haven’t discovered any that talked about sanding all the wood and plywood before applying any T88 for assembly. If this is not something that would cause any issues when gluing pieces together I would prefer to sand the wood when it is flat and not assembled and not have to sand the wood when it is assembled and the difficulty of sanding goes way up.  Are there any issues with sanding the wood prior to assembly and after the gluing is finished go ahead and varnish it?

Any and all thoughts on this are certainly welcome.

Like Leonard said,”70% weight difference between non-sanded wood and sanded wood that is varnished.  The difference is like painting grass or painting a concrete side walk.”  This certainly makes sense to me.

So, how about sanding the wood before assembly and be done with it and then varnish?

Feel free to chime in on varnish and glue too.

Sparrow

[s johnson]

Sparrow,

I'm not an expert on either glue or varnish but as I understand it on gluing, you don't want to sand before you glue due to the fact that you could actually be filling up the grain of the gluing surface with the sanding dust. I suppose if you did sand before  gluing you could always vacuum or blow it out with air. Just how I understand how it's done.

On varnish, you can sand but clean with a tack rag as well. You can put another coat of varnish over the two part EV-400 epoxy varnish with out sanding if with in 7 days but after that it should be scuffed of again .

Again I'm not an expert on either subject and open for discussion..

Scott J.

[Poorman2]

John, I lightly sanded the leading edge sheeting with 220 grit on both sides before I glued them on. I also taped them off where they would be glued, then varnished the inside. I did not sand the ribs before gluing the wing together. I wish I had. Now I have to go back and sad before I can varnish. It would have been easier before installing them, but I didn't have to worry about getting varnish where they would be glued either. Joe Spencer mentioned thinning the EV-400  varnish pretty good to save weight. I also have used T-88. Double gluing every thing. Good luck. Randy

[s johnson]

For new wood application with the EV400 varnish it says to thin 50% the first time. It really helps it soak in.

Scott J.

[rockiedog2]

some of the plywood has a waxy surface that glue won't stick to. sometimes it's hard to see so i sand every piece both sides with a vibrator sander and 100 grit as soon as it comes in the shop and mark it as sanded so i know there's no doubt. as some said there is danger of leaving dust in the grain. i blow and brush it at the same time til i'm sure i got all the dust off. i'll take the small amount of remaining dust(if there is anyi don't think there is) over wax any day.
AC43.13 prohibits sanding of softwood prior to bonding but goes on the say that sanding of some hard plywood is allowed to remove wax etc. then it describes a wetting test to ensure the surface will take glue. its all right here guys and free



the old version that i use outright said to sand hard ply for a good glue surface. they have revised that to the current mealy mouth version that you have to read between the lines for.

the final test is of course your frequent destruct tests. i know of one set of Pitts ribs that had to be destroyed because he didn't sand the wax off of the ply. the gussets would pop off with a knife blade pushed under them. a winter's work...

[Sparrow]

Hi Scott, Randy and Joe,
 
Thank you for the information.
 
I am going to read what Joe linked us too first.  I will be buying some T88 and making a bunch of samples and gluing them up and testing to destruction using shear, tension and torsion loading to failure.  If you want me to document and post my results please let me know. 
I intend to test plain wood that is free of wax, sanded and blown down and vacuumed wood and sanded, blown down and vacuumed and then wiped down with a solvent and left to dry.  I am not sure if wiping with solvent will lift the fibers and again introduce the heavy wetting action that will cause a lot of varnish to stay behind or not.  I am going to find out.  I have found that in finishing furniture to a glass like finish it was always important to put on the first coat of polyurethane and then sand after it had hardened.  All the wet fibers would rise to the top and stick through the surface and the second sanding with 0000 steel wool, tack cloth and apply a finish coating of poly always did the trick.
 
If there are any recommendations regarding the most reasonable place to purchase T88 and the EV-400 please let me know?
John
 

[Sparrow]

Just finished reading AC 43.13-1B, CHAPTER 1. WOOD STRUCTURE
SECTION 1. MATERIALS AND PRACTICES. 
I have always found that reading government imposed articles can be either quite useful or consistent with a bag of 10-10-10 fertilizer.  You have to spread it around and wait and see if things grow.  Until then, it is corrosive and quite useless.
 
I refer to this link and I am in agreement with what is expressed there.
http://www.2wings.com/m12/faq/wingconst/wingconst.htm#08/04/02%20VARNISH,%20GLUE%20&%20EPOXY
 
The conclusion of one aircraft restoration expert is summarized by stating:
To sum up:
 
 We use weldwood plastic resin glue on certified airplanes.
 We use T88 on experimentals and will use it on certified airplanes if it gets approval.
 We use EV400 epoxy varnish on all wood structures applied after construction is complete.
 We use the polyfiber covering system. KK
 
I am still working on finding out if anyone sands once before assembly and then uses the EV400 varnish.
 
John

[Sparrow]

A piece of information from 2wings.com I am copy and pasting concerning EV400 and T88 and how they interact.
 
I used the Polyfiber EV400 on mine, stuff works great.  One thing to keep in mind is epoxy won't adhere to anything that's been sealed with EV400 which is why it's recommended you do your varnishing AFTER all the ribs are on the spars. However one good tip is to varnish under your gussets before putting the ribs to spars, again just make sure you don't get varnish where you still need to glue. 
 
I liked the idea of putting varnish under the gussets before installing them to the spar.
 
I was also interested in putting on the leading edge plywood and tilting the back of the outboard rib up by 1” to have the wash out twist already in the “D” section created by the spar + plywood to prevent any dimpling when the wings are rigged.  Has anyone done this and is this advisable?  John

[Sparrow]

Well, here is another piece of information I was able to find.



Andy at 2wings.com is telling about the use of System 3 coating with T88 and the T88 will adhere to the clear coat if you scuff it up with sand paper.  If you use the EV400 this is not a possibility.  Just another option to consider that is out there. John
 

From the 2wings.com blog

 
I just received a FAX reply back from dick Anderson, the head tech at System Three.  MEK has no effect on either cured T-88 or their cured "epoxy clear coat".  So anyone using epoxy clear coat from System Three rest easy. I had this as a verbal several years ago, before I decided to use it. But now I have it in writing as well.
 
 Kevin, and anyone else doing "commercial" covering: I understand you have no choice but to use all "Polyfiber" products so you do not have the possibility of finger pointing if something goes wrong. You have to do it.
 
 For me, that is not a concern. I like the deep penetration of 2 coats of the epoxy clear coat, and the bonus that it strengthens the wood significantly in the area of the penetration. 

 On "thinning" T-88 or their epoxy clear coat----do NOT thin by adding anything. PERIOD!  The correct way to "thin" T-88 is with a LITTLE heat to each component (if you apply heat to the mix, you will accelerate the cure - yes, heating the components will also accelerate the cure), but not much over about 100 degrees F.
 
 Epoxy clear coat from System Three is almost water thin and requires no thinning.  Andy

[rockiedog2]

>>>>ndy at 2wings.com is telling about the use of System 3 coating with T88 and the T88 will adhere to the clear coat if you scuff it up with sand paper.  If you use the EV400 this is not a possibility.  Just another option to consider that is out there. John

seems to be a little controversy about sanding/gluing the EV00. Years ago I talked to one of the tech guys at Poly Fiber about gluing over T88 with T88 and also over EV400. He says its fine as long as we rough it up with coarse sandpaper. 100 grit is what I use. And as I recall he said the EV is the same base material and yes it does penetrate deeper depending on the variable factors but roughing it up will provide the same mechanical bond as over the T88. Of course we have to be sure and remove all the sanding dust as much as possible. Well, anyway, over the years I've glued over the top of a lotta T88 and EV400 as well. It often can't be avoided when doing repairs, mods, and fixing screwups. I never have liked to do it and am careful about it when I do but do it and press on and never a problem but tomorrow is a new day. If I had any doubts about it I wouldn't do it. Experience is a considerable factor when nibbling around the edge of what will work or may cause problems. Your call as always.
 

[grdev]

If I'm not mistaken, I think my friend at EAA 431 used the System 3 adhesive and soaked each rib in a pan of it. These guys do this for a living, restore Vintage aircraft. He showed me this about five years ago so the details are a little fuzzy now

[Sparrow]

Thank you.
 
I am going to purchase some of the System 3 Clear Coat and T88 and run some testing.
 
I am going to make samples and weight them after sanding them and record this information.
I will then coat them with the Clear Coat and allow it to cure.
I will again weigh the pieces.  I am going to use a gram weight scale for improved accuracy.
I will then rough the surface with some 100 or 120 grit sand paper using the vibratory sander and vacuum off the dust.  I will then T88 the pieces together and do destructive testing after a 72 hour cure at above 70 degrees F.  I will weigh all samples prior to testing to determine how much weight was added by the product.
 
I would like to do this same test using wood that isn’t sanded and glued and then coated with EV400 and test samples sanded and glued with T88 and pulled to destruction.
 
I will be tabulating the results and post them when I have them.
 
Right now I like the Clear Coat and the T88 over pre sanded surfaces but I have concerns regarding weight added due to the Clear Coat being a solids based epoxy.  John

[Jlwright]

I want to thank you for all your efforts and for letting us all in on your findings. This is very good information for new builders.

[grdev]

John I'm not so sure you want to sand the part you are going to add glue to. I would ask an expert, but I've been told it is not a good practice as you will leave abrasives embedded in the wood from sanding.

[Sparrow]

I agree Noob,
The regulations advise against it and I am getting different testing results from other groups I have been viewing.  The system 3 technical fellows are stating their product, when using Clear Coat on sanded surfaces and then roughing up the area where T88 will be used will always cause the destructive test to take place in the wood to the depth of the Clear Coat penetration point in the wood.  I am going to find out by testing.
After sanding I will blow the area with an air gun set to 90PSIG and then I will vacuum the piece before using the clear coat or doing a T88 adhesion test only and then test to failure after fully cured.  I can see, in my minds eye, the filling in of areas where the adhesive needs to absorb into to achieve an acceptable bond.  I hope the air pressure and vacuuming will again open the surface up to good adhesive bonding.
I am interested in empirically clearing up the confusion regarding this because this is being talked about on many build sites.  John