USING WATER-BASED POLYURETHANE VARNISH (WPV) FOR GLASSING
BY: SKIP POTHIER
WITH A LOT OF HELP FROM HIS FRIENDS
This article focuses on the construction techniques used, but not design considerations, as the design of a float will vary with the aircraft it is intended for and the whims of the designer/builder. The particular floats I built were based on the 36 inch Carl GoldbergÓ design, intended for the 40 size Carl GoldbergÓ J3 Piper Cub.
As with any building project being undertaken for the first time, some basic research is necessary. This usually means reading up on articles like this and talking to people in the hobby who have experience in the various aspect of your project. Before I undertook to build my first foam core floats I spoke to David Summers of the Toronto RC Club who had been experimenting with using WPV instead of fiberglass epoxy to apply fiberglass. I also received good suggestions from Jim and Don Moss of the Whitby Aeromodelers who have built and flown many float planes. Then I went to see Ron Brownell of the Oshawa RC Club who has built his share of foam core floats in his time and who happened to have the precut foam cores I was looking for. He explained step-by-step how he produced the fine finished product he used as an example. I bought my foam cores from Ron.
These conversations, as well as observations and discussions with other float flyers during various events, were added to the knowledge gained from reading relevant articles in the various RC magazines and web sites. Together, they served to finalize my approach in constructing my first foam core floats.
A list of the materials and tools I used is at the end of the article. Please familiarize yourself with them as you would in any building project so that the instructions are easier to follow. As with any project, before you start building make sure you have everything you need and that everything is in good shape. This will save a few trips to the hobby shop.
The Astep@ in the bottom of the floats needs structural strength as it takes the most wear and tear. This is accomplished by applying a piece of lite-ply the exact size and shape as the step, to the rear of the step. Cut it slightly larger than the step so that it can be sanded to the exact shape of the foam core after installation. Epoxy works well here. Don=t forget to punch holes in the gluing surfaces to give the epoxy some extra grip. I use a small wooden dowel with a small, sharpened nail in the end for this purpose.
Repeat the process to cut and glue the lite-ply pieces to the rear of each float. These will hold your rudder hardware.
The cores I bought had a pine insert (spine) that ran from the rear of the float to several inches ahead of the front gear mounting point. The foam cores were routed out to accept them. I drilled the spines for lightness. To prepare for the mounting hardware, I cut out squares of lite-ply the size of the mounting hardware. These were then counter sunk into the spines at the gear mounting locations, allowing for the depth of the balsa sheeting, so they would be flush with the surface of the balsa when it was applied. DremelÓ out the spines to accept the lite-ply. Do not attach the lite-ply at this time.
I made sure the spines were flush with the foam and made a paper plan of their location including the lite-ply squares so as to know the location of the mounting points after the balsa is applied to the top of the floats. Epoxy your spine (but not the mounting hard points) into the top of each foam core. Don=t forget to poke those holes to allow the epoxy something extra to grab onto.
To keep the spine anchored in the foam and prevent it from creeping under load I epoxied two 1/4 inch dowels vertically through the spines into the foam. They were inserted 2 inch ahead of the front mounting point and the same distance behind the rear mounting point. They were half the depth of the foam in length.
We are now ready to apply the balsa to the top and sides of the foam core. Some builders prefer to apply the balsa on the sides with the grain running vertical to the floats. This is done to provide extra vertical strength. The foam cores will flex under load, causing stress cracks to form with time. I had done test coverings to compare the WPV to the usual epoxy on fiberglass. I found that the WPV provided some flex as opposed to the stiffness of the epoxy when hard which would be more inclined to crack. I concluded that the flex of the WPV would lessen its tendency to crack under stress and allow me to apply the balsa sheeting with the grain running lengthwise. This also saves a lot of building time.
I use LePage'sÓ water-based contact cement for this purpose as it is easy to work with and has no odour. It spreads easily with the foam wedge brushes. Be careful in applying the balsa as it is CONTACT cement. Once on, the balsa cannot be removed without damage. The contact will be affected by humidity. The drier the air the quicker and harder the contact will be. Please be careful.
Cut all balsa sheeting slightly oversize to allow it to be sanded to the shape of the float after it is glued on. Be sure to allow for overlap of the lite-ply pieces already installed and for overlap of the side pieces by the top piece.
Apply the balsa to the sides first. Glue and apply both left sides on each float. When they are set, glue and apply both right sides. When dry, sand all of the edges to the shape of the float.
The balsa sheeting for the tops can now be prepared.
Using your plan made in step 3, mark on the floats the exact location of the squares of the mounting lite-ply. Cover those areas of the foam and spines with wax paper to prevent the glue from the balsa sheeting from sticking to them. Using contact cement, apply the balsa unto the tops of the cores.
Once they are set, again using the paper plan made in step 3, cut the balsa out from over the mounting points. The wax paper underneath will allow you to pull out these squares of balsa without damaging the cores. Epoxy in the lite-ply mounting squares. You will have to remove some of the foam to get them to fit exactly due to their being countersunk onto the spines. If you did it all right, they will be flush with the surface of the balsa and you have a good hard point to attach the mounting hardware.
The application of the micro-ply to the bottom of the cores takes a bit of care as it goes on in three pieces and all of the joins must be as perfect as you can make them.
The first piece to apply is on the rear portion of the float behind the step. It must butt against the lite-ply on the step and extend over the lite-ply on the rear of the float. Cut the sides and end of the micro-ply slightly oversize so it can be sanded to shape after it has been glued on. I found that the micro-ply is flexible enough to be applied in one piece over the curve of the bottom. The contact cement worked well here as well.
The compound curve of the front end of the floats requires that the micro-ply be applied using separate pieces for each side. The trickiest part here is to get the curve of the Vee bottom accurate. I made a paper pattern to trace unto the micro-ply. Again the micro-ply has to be cut slightly larger than the pattern so it can be sanded to the shape of the float.
When you are satisfied with the fit, apply the lite-ply to one side of the float using contact cement. Repeat for the second float. When these have set well, it is necessary to sand the lite-ply on the Vee of the float to the exact shape and angle of the uncovered side. It will be necessary to very carefully sand along the raw foam on the other side of the float. This will leave a beveled edge on the applied half of the micro-ply along the Vee to accept the micro-ply applied to the other side. Now repeat the process for the other sides. After they are set you sand the overlapping micro-ply along the Vee to meet the edge of the first side.
All this sanding takes a lot of patience to get a good fit and finish along the Vee but is well worth the effort. I found that using an eleven-inch aluminum sanding bar with stick-on sandpaper very useful in this process and in fact with all of the sanding.
More sanding. Before applying the nose cones to the tips of the floats it is necessary to sand all surfaces of the floats to get them close to their final shape and finish. This will make it easier to shape the nose cones as the lines of the floats will be well established.
I found using the sanding drum attachment of the DremelÓ tool very useful in sanding the excess of the lite-ply down to the point where it could be sanded to shape using a sanding block.
Carve the nose cones close to shape. The grain should run horizontally. Epoxy the nose cones to the front of the floats. Don=t forget to punch some holes into the gluing surfaces to help the epoxy grab. Once the glue is dry, sand the cones to their final shape. Be sure to follow the upward Vee curve of the underside of the floats to ensure they will give a good water parting action.
To provide some protection against Adocking@ damage to the cones, apply several coats of thin CA to them. The grain running horizontally will help it soak in and harden them.
Now check your efforts so far and do any final sanding/filling necessary before you start to apply the fiberglass.
BUT FIRST! It is better to add any balancing lead to the floats rather to the plane so that when you take the floats off the plane is still balanced for its regular gear. It is also better to do this now as it will make disguising your balancing efforts very easy.
Attach all your hardware including rudders and mount the floats on the plane. Check the plane=s balance with a balancer - not your fingers. Your floats will normally be tail heavy and need weight added to the nose.
With the plane on the balancer at the required balance point, place lead shot on the nose of the floats to determine the amount necessary to provide the proper balance. Drill an appropriate size hole in the front of the floats behind the nose cones. Place the lead shot in the holes and epoxy it in. Recheck the balance. Take the floats off the plane, refill the holes as necessary and finish with a sanded flush piece of balsa.
Remove your hardware and proceed. Keep track of your mounting holes by sticking a pin in them after each of the following steps.
Water-based products tend to soak into balsa causing it to swell up. To help prevent this, give your floats a couple of light sprays of the WPV to seal the pores in the balsa and let it set. While waiting, cut your fiberglass to size. Cut separate pieces for the nose, steps and tail pieces to make it easier to cover them smoothly. Also cut separate pieces for the front and rear sections of the bottom. Cut the top and sides all in one piece. Leave at least 2 an inch overlap all around. More is better if you want room for error. It can always be trimmed.
The basic application is the same for each piece. Using a foam brush apply a coat of WPV and let it set to slightly tacky. Apply the cloth as smoothly as you can. Once in place apply another coat of WPV. You will find you can make any final adjustments and smoothing of the cloth during this process.
Apply the glass in this order: Step, nose pieces, tail pieces, top and sides, rear bottom and front bottom; allowing the WPV to dry at each step. The step pieces should overlap onto the front and rear portions on the bottom at least 2 inch. The nose pieces should reach back about 2 inches all around. The tail pieces should overlap all around at least 2 inch.
The top and sides should overlap at least 2 inch all around. At the nose they should go as far forward as possible over the glass already there without getting distorted where the nose cone narrows.
You could choose to apply the nose piece glass after the top and sides, but as they do not take a lot of water pressure it is really not necessary. The bottom pieces should overlap up the sides well. This is where most of the water pressure will be on take off and landing. You want to make sure the glass here is applied tightly over the edges to prevent the water from attacking any weak spots. Make sure you have at least 2 inch of glass up the sides.
If you can, let the floats dry in the sun at this point. You want the WPV set well before the next step.
The objective at this stage is to prepare the glass for its filling coats of WPV. Start with some 150 grade sandpaper and, using it lightly and loosely, brush off the bits and loose ends of the glass. Then, using a 300 grade open wet sandpaper, give the whole float a very light wet sanding to take off any noticeable roughness. Do not overdo this - leave the glass as untouched as possible. Do pay some attention to the areas where the glass overlaps.
Any Apinholes@ in the weave of the cloth will haunt you when you reach the painting stage and they refuse to fill in. They are also potential leaks once you get the floats into action. Water under the glass and into the balsa will cause swelling that is not easily fixed. This is resolved with some weave filling. Using three parts WPV and one part baking powder (not soda) mix a batch to paint both floats. Apply this liberally with a foam brush. Allow to dry thoroughly. Now wet sand the floats with 300 grade open wet sandpaper. It will not take much pressure to achieve the result you want. This is one of the advantages of using WPV. It dries quickly and wet sands extremely easily. Other than necessary to clean up loose ends, do not take off any glass! Now wipe it down with a damp cloth and allow to dry.
Repeat this technique until you are absolutely sure you have left no pinholes. You will find that with each coat (usually 3) you will get a smoother and smoother finish. Remember to use the sanding bar to keep things flat. When you have reached the point that you can run your hand over the entire surface of the float without feeling anything and you can see no pinholes when you look sideways at a flat angle, you are ready to apply the final coat or two of WPV. This last coat is pure WPV and will keep the baking powder from reacting with the final coats of paint. Note that while you may be able to see where the glass overlaps you will not be able to feel anything but a smooth surface.
Having worked hard to get the surface smooth we now have to give it a light sanding to create a surface the paint can adhere to. For this purpose I found that 3M Scotch-BriteÓ Sanding Pads are ideal. They will roughen the surface without removing a lot of the paint and are easily handled as they conform to the surface being worked on.
A general rule in painting is that you can apply an oil-based paint over a water-based paint but not the other way around. I like the look of TremcladÓ aluminum paint on floats as it resembles the look of real floats. Using the spray can version and following instructions, give the floats a light coat. Let this dry to the point it is dry/tacky and give them a second light coat. Let this set until hard. One drawback to TremcladÓ is its long drying time. Once this has hardened, give it a light rubdown with the 3MÓ Sanding Pads. Then spray on a regular weight coat and let it set thoroughly.
Sand this coat with 300 wet open sandpaper. Wipe down with a wet cloth. Repeat this rub down, painting and sanding until you are happy with the result.
TremcladÓ paint is reasonably fuel-proof, especially if allowed to set thoroughly. To add that extra insurance I spray on two coats of Tremclad VarathaneÓ. Use the regular version, not the water-based, for this purpose. I use the semi-gloss as it keeps the look of real aluminum. The high gloss gives a plastic look to the finish and the matte looks a little dull.
One coat - 2 if you need it - of black high gloss on the nose cones and you are done.
TIME TO FLY
Remount your floats, check you balance again and go flying.
It is usually a good idea to taxi test the plane on the water and recheck all your mounts and other hardware before putting the plane in the air for the first time. It is tricky to land a plane with the floats in other than their intended position.
Floats take quite a beating. Examine them thoroughly on a regular basis.
Before the beginning of each summer=s flying I check closely for stress cracks and respray them with the oil-based semi-gloss VarathaneÓ. If there is some indication of stress cracks, a good sanding and repainting with new coats of TremcladÓ and semi-gloss oil-based VarathaneÓ will waterproof the floats again.