By: Bill Miles
There is no doubt that more than ever, that Pro-bond glue is probably the most controversial none-R/C product to come along in awhile (at least on the I.M.A.C. mailing list). It seems everyone has their own technique for using this glue and everyone has their theories. I will give you my method and opinion on the glue and also go through some simple tests between odorless C.A., epoxy, 3m 77 spray, and contact cement. I will cover odorless C. A. because any of these glues can also be used for capping stabs and wings.
First, Probond is a product made by Elmers. It is found in most do-it-yourself super stores like Lowes, Home Depot, and even Wal-Mart. It comes in a black, white and gold bottle. It comes in two sizes, oz. and the large bottle oz. (in the long run-cheaper if you do a lot of wings and I do). It has the consistency of pancake syrup. It is a little thicker than some of the finishing resins out there, so it doesn’t soak into the wood or foam quite as quickly. It is water-soluble so it’s easier to work with than epoxy. With epoxy I usually have to have a bottle of alcohol, latex gloves and baby powder standing by (that’s right-baby powder). Along with epoxy, Probond is easy to take off of your hands with baby powder and if you just don’t want glue to ever touch your hands, I guess you may want to still use the gloves. But at any rate, it’s easier to wash off. As with epoxy however, you never want to let this stuff spill on your wood building surface. You will never scrap it off or pull it up without pulling up some of your building board.
Everybody who has used it out there has his or her own theory for using it. First of all, the directions on the bottle say to wet the surface before you use it. I have found that this is not necessary. The humidity here in S.C. is usually enough to negate the use of water. I first start with the usual for sheeting wings. A large (depending on the size wing panel) flat clean building surface, wax paper, the glue, masking tape, and whatever form of weight that you want to use. I use several sheets of particleboard leftover from jigs that I built for fuselages.
It is heavy in several sheets and it disperses the weight evenly over the entire wing surface. I usually take some form of sticky backed sandpaper and stick it to one part of my building surface in a long sheet. Great Planes makes great sandpaper for they’re sanding bars. I use this to edge sand the balsa so that all the sheets are even. The best method that I have heard of is to use T-pins to hold all of the sheets together in one stack and then run the stack up and down the sandpaper to make an even edge. After edge sanding (if I have to) I then tape the sheet together using masking tape, with the edges of the sheets butted together.
You then have an option with Probond (and this is probably the neatest part). To glue or not to glue, that is the question! What I mean by this is either you can edge glue the sheets or leave them taped together (believe it or not)! Do all wing skins in this fashion. Lay down the first saddle. You now have an option of putting wax paper down or not. If you do not glue the sheet together, you should probably wax paper the entire saddle. There is a chance of the glue coming out between the sheets. It is a very slim chance and it wouldn’t ruin the panel if it did, but it just adds time to the building when you have to pick or sand foam off of your wing.
Now lay in the first wing skin. You can put glue on the skin before or after you do this. I do it after. I usually start at one corner of the skin and then go back and forth diagonally about 2 inches apart at a 45 deg. Angle all the way lengthwise down the wing. Then I go back in the opposite direction. This creates a waffle pattern on the wing. I then take a squeegee or similar item and smooth out the glue.
Keep in mine that the more weight that you can put on the wing, the less glue you have to use. I have done tests to prove this theory. I then lay the panel onto the wing skin. Put glue on the other skin in the same fashion and lay on top. Put more wax paper on and lay the other saddle on top.
I then place the particleboard on top of the wing. I usually use 4 sheets the same exact size of the panel. I then place everything heavy that I can find in my shop on top of the panel. I even have old plastic soda bottles that I have filled with water to place on top of the particle board. Now, because of the wax paper you can actually move around the panel in the saddle to get it lined up properly (another advantage of fully sheeting the saddle with wax paper). I have only let the panel dry for about 3-5 hours before pulling up and cutting the sheeting, but I usually let it dry overnight.
I have also used this glue for capping the wing off. There is one word of caution however, excess. Don’t put a lot of glue on your caps. I usually put the glue on and then use my finger to squeegee most of the glue back off. If you don’t do this, the glue has a natural tendency to foam-up and may push the caps off of the wing panel to far; creating a wing panel that may not be square on the tip, etc.
Let’s talk about what tests have proven. First, Probond has a tendency to seep down into the foam about &Mac189; inch into the foam. You will not get this with contact cement. Epoxy seeps down into the foam but not as far and is heavier. Spray 77 is easy to use and light but tests have shown that in the sun and on larger planes it has a tendency to delaminate. Obviously because of time-to-work reasons, odorless C.A. is not good for sheeting large surfaces such as wings but may be better for wing tip capping and small areas.
Keep in mind that the object here is to use as little glue as possible and as much weight as possible. Remember that for a 1000 sq. in. wing panel, that 1000 lbs. is only 1 lb. per sq. in.!!!! Remember also that you must put in your phenolic tube support before sheeting your wing! Yep, that’s right I have done it. Not on a customers plane of course. Another tip, have all of your items that you will need ready and be totally prepared before starting your wing sheeting project. There is nothing worse than starting and not realizing that you don’t have enough weight ready or that you don’t have enough glue. I hope that your next project goes well and remember, “We build, You fly!” If you have any questions send us an email.