Balsa Skinning

[portfoxdesign]

Its probably been asked before and wont be the last. I have searched the threads and havent been able to find particulars on this.
I want to cover my foam cores (not EPP) with balsa skins using PVA glue. I have read the instuctions with my aircraft and it makes sense until the manufacturer says applying water based glues to balsa will make them curl.
My questions is (starting on the top section of the wing), if i spread the PVA along the foam core and along the balsa skin, attach the skin to the foam ASAP, place the core and balsa back into the jackets and apply pressure eg phone books. Will that be the right way to apply PVA in this situation?

The manufacturer went on to say that you can wait until the PVA has cured a little (which curl will straighten) then apply the balsa to the core.

I know this is th old way of doing this procedure, but i like to work slowly and surely to get a straight wing.

Any thoughts?

[Aten W Arthog]

What I heard is that PVA makes a great thermosetting adhesive method: you coat both sides with a thin layer like it ws contact adhesive. Let dry, then hit the balsa outside with an iron while pressing down. Supposedly the foam will not melt if you keep the iron moving and don't park it in one spot too long. Try it on some foam and sheet scraps, see if it's for real.

[portfoxdesign]

Thanks for the advice, will have a go at that.

[Ollie]

If you have the schucks left from cutting the cores, they can be used in a wet layup. Apply the PVA to the balsa skins and scrape off any excess. Assemble the skins to the core and tape in place with masking tape. Put the core between the shucks with waxed paper to prevent any stray glue from sticking the balsa to the shucks. Place the assembly on a flat surface with a flat piece of plywood on top. Then weight the plywood down with bricks, water filled paint cans or what ever you can. The more weight the better up to about five pounds per square inch. Keep it under pressure for at least 24 hours.

Polyurathane wood glue works even better. It cures by catalytic action with moisture. The glue foams slightly while curing, forcing itself into the crevises between the foam beads in the core for a most tenacious joint. The proceedure is the same as with PVA but just before assembly, the cores are misted with water. The damper the cores the faster and foamier the cure but, don't over do the mist. Also, the pressure only has to be maintained for a couple of hours for the polyurathane to fully cure.

[portfoxdesign]

Thanks Ollie, i already have the PVA, so will have a go at that. My next model also has foam cores which are smaller in span. I will try the Polyurathane on those.

Cheers

[Ollie]

I forgot to mention that the balsa sheets don't have to be joined at the edge when using polyurathane glue. They just have to be taped together with masking tape before assembly. The edges will be glued together as the polyurathane works its way into the edge joint while the core and skin assembly is curing under pressure. A disadvantage of the polyurathane wood glue is that is about 3 or 4 times more expensive than PVA. An advantage of the polyurathane wood glue is that it sands well. The more it foams and the quicker it cures, the easier it is to sand.

[Antares]

How it compares with PVA or epoxy, weight wise?

TIA

[Ollie]

The three alternatives mentioned, PVA, Epoxy and Polyurathene wood glue are each very highly dependent on how they are applied. They need to be applied as thinly as possible and thoroughly weighted down so as to close the microscopically thin glue joint.

Epoxy weight can be minimized by using water thinned polyurathane warer based varnish on the wood first. It seals the wood against excess epoxy penetration and thereby saves weight. The final results are dependent on how much the varnish is thinned and how thinly both the varnish and epoxy are applied.

The PVA can save weight by applying it to the wood, scraping most of it off and then doing a wet layup under pressure. The water in the PVA evaporates and only the solids are left.

When the polyurathane wood glue is applied to the wood and scraped off and the foam misted with water, the glue expands a little by foaming and penetrates both the pores of the wood and the crevices between the beads of foam. The volume of the foamed glue is greater but the density is less.

The final results are more dependent on the technique used than the type of glue selected. Any one of the three glues can give heavier results if inappropriate construction techniques are used.

One of the lightest and best adhesives is the old formulation of 3M77 spray contact adhesive. If a light spray is applied to both surfaces, allowed to dry for 20 minutes before assembly and the assembly kept under pressure for at least 24 hours, a very strong, light and permanent joint results. If it isn't kept under pressure long enough the joint may fail later.

[jgleigh]

Another method you might want to consider is using
double-sided picture mounting tape. The advantages are indefinite working time, light weight, no mess, and an extremely strong bond. The disadvantage is that you have to have the skin aligned right the first time- once it goes down it is not going to move. It's my preferred method these days when I have to skin a wing.

Just my 2 cents.

- John Leigh

[uscra112]

I have been using a water-based contact cement for this task. yes the wood curls.

It's not a problem.

A simple vacuum-bagging gadget will eliminate any problem with the balsa curling, and will guarantee you a very straight wing with a perfect, uniform bond, to boot.

I have built a flat, fairly airtight box, about a foot wide, and a couple of inches thick, and longer than any wing I plan to do. Made from 3/4" hard-surface particle-board. Has reinforcement ribs inside to keep the top flat. On the side is a hole into which a vacuum cleaner hose fits. On top, a few strategically placed holes (1/4" is enough). This cost me about $20.00 in material.

I actually use this as a vacuum chuck to hold the blanks while I wire-cut them, but that's another story.

On this, lay down the lower shuck. Now stick the pre-cut-to-size lower skin in the shuck, using a few bits of double-sticky-side tape. Paint the core and lower skin with your glue, and lay in the core, then more glue, then the upper skin. I stick a couple of pins right thru the whole mess to make sure it stays lined up in the next step. Lay a sheet of plastic over all this, and tape the edges to the sides of the box for a seal. ( not critical - it's almost selfsealing anyway.) Do NOT use the upper shuck!

Now apply the vacuum cleaner to the box. It will pull ALL the curl out of the balsa, and pull everything down flat to the box-top reference surface. Leave it for a while. The water-base glue I've been using needs about 4-6 hours.

Now, about a dozen people will post saying you'll kill your vacuum cleaner this way. Not so, if you use a shop-vac type. They do not depend on flow thru the hose for motor cooling. I bought an ultra-cheapo called a "Stinger" at my local Home Despot, which is dedicated to this job alone, and it has lasted for many wings. If it dies tomorrow, it will have cost me about $1.00 per airplane. Only downside is it's noisy, so I have to set it up at the other end of the house.

Let me put in a plug for this West System epoxy - foam safe, does not stink like polyester resin, West sell neat dispenser pumps for it, and it cures tough, yet fairly resilient, even in thin films. It wet-sands very nicely. A coat painted right over bare blue foam, (with no glass cloth at all), makes a pretty tough skin all by itself.

[jlk]

Vacuum bagging gives me the best result. I have used epoxy and never have a problem with it soaking into the balsa. The trick is to not use the laminating epoxies that are really made for fiberglass. Use a thick epoxy like Hobbypoxy formula II. Apply it to the skins and then scrape off all you can with a playing card or credit card. The skins should look dry when you have the right amount left.

You do not need to glue the sheets together with this method either just tape them. This method only works with vacuum bagging or with lots of weight on the core beds.

Jordan

[uscra112]

jlk - you're keeping the amount of epoxy to a minimum to keep weight down?

Is this really that much of an issue?

Or are you trying to control bond-line thickness?