Recommendations for preparing a nearly-finished surface for a structural bond.

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cattflight

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Spokane, WA
Pursuant to some of the ideas offered here, I am looking for some feedback.

Without asking you to read the entire thread in the above link, suffice to say some smart folks recommended adding "blisters" to the outside of a per plans Cozy fuselage to solve my interest in increasing elbow and hip room in the front seats. Whatever my decison might be in terms of how extensive I want to modify the per plans walls to accommodate this, I am looking for feedback/recommendations on how best to prepare a nearly-finished exterior surface for a glass-to-glass or foam-to-glass structural bond. Not looking for recommendations on how to compensate for the loss of structure in such an idea, only how best to prepare the surface for the modifications.

Here's why: I may have access to a nearly-completed airframe project, but the surface is, essentially, fully-prepared for the first round of primer. (read: completely filled and smoothed with micro) None of the underlying glass is visible and there's nothing there to "bite" a new layup. So, if I were looking to make a modification to such a surface, what would be the best way to prepare this surface? I have seen guys sand-blast (at nominal force) a composite surface to prepare for finish coat while others have sanded. The argument for sand-blasting is great because the material gets into the crevases of the glass weave without destroying the peaks in the fabric, but I don't know whether that would work for a nearly finished surface. Does this even matter? Is scuffing the surface enough? After all, a peel plied surface is pretty darn smooth also.

Feedback welcomed!
 

autoreply

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Sanding is the only way I use for repairs (essentially the same), as do most repair shops. I can see the theoretical argument for sand-blasting, but aside from some practical problems, I wouldn't be SURE about it's structural strength, which is by far the most important consideration.

Peel ply can result in other chemicals on your finished part, weakening or ruining the structural bond.

In most repairs, you sand through several glass layers. That's not a problem, because you will lay up new glass, which takes over the load of the original glass. By having a gradual decrease in sanding depth (just surface until completely through), there's a gradual transition from the "old" to the "new" glass.
 

delta

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Brookside Utah
"The argument for sand-blasting is great because the material gets into the crevases of the glass weave without destroying the peaks in the fabric, but I don't know whether that would work for a nearly finished surface. Does this even matter? Is scuffing the surface enough? After all, a peel plied surface is pretty darn smooth also."

I wouldn't sand through any layers were I you. Sandblasting sounds like a good idea if it leaves the glass and eats the resin. What would it take to get it cleaned completely afterwards. Air and MEK? Don't wait to long to do your layup after preparation. Maybe have 2" overlap instead of 1", and see if you can peel it off when it sets up.

Rick
 

orion

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I wouldn't sand through any layers were I you. Sandblasting sounds like a good idea if it leaves the glass and eats the resin. What would it take to get it cleaned completely afterwards. Air and MEK? Don't wait to long to do your layup after preparation. Maybe have 2" overlap instead of 1", and see if you can peel it off when it sets up.
Whether you can sand through the glass material or not really depends on the application, the anticipated loads and the type of materials you're using in doing the repair or modification. As such, this is not simple to answer. In general, it is not a good idea to sand "through" any layers since that will create a stress concentration (especially in a thin layup) that could eventually initiate a crack, or even peel from the core if sufficient motion exists int he immediate area. However, for fully cured composites you do ned to expose the glass of the topmost layers which as you suggested, can be done with some form sand blasting (actually crushed walnut shells are probably the best blast medium for this). Fully cured resins will have completed the joining of the vast majority of free molecular chains so you will not be able to depend on the chemical bond - you will need a mechanical bond, which will be accomplished through the contact with the glass.

Some sanding is possible but do not sand through the outer layer - after all, it will most likely be structural But you can sand down enough to expose that outer layer's fibers. You ideally want to remove any fill material and get down to the glass surface, and then just a hair into it.

Cleaning? You can use air but only if you know it's dry and does not have any oil or moisture in it. At best you can blow off the sanding dust. After that point though you do not want to use compressed air. You can use a vacuum and then complete the cleaning with MEK or Acetone. Try not to use the typical hardware store variety of these (unless you have no other choice) since these retail products may include some contaminants. Try to get your materials from a professional paint shop or composite supplier.

As far as trying to peel it off after appication: Don't! At least not too soon. Remember that any resin will require quite a bit of time to achieving full cure, especially in a secondary bond application. Typically full cure will not be achieved for at least three months, although some slower set materials may actually not reach full cure for more than six months. The instructions may say full cure in eight hours (or whatever) but that's usually a handling strength. It's not a structural service strength.
 
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