December 22, 2023

Basic procedure for applying fiberglass cloth to a model aircraft

This is Part 3 of a multi-part article describing how to apply fiberglass cloth to a model aircraft.

Part 1 of this series describes advantages of using a fiberglass base and recommended cloths to use.
Part 2 describes resins and squeegees.

Before you start

Some people will tell you to use spray glue to hold the cloth in place. The idea is to spray a very light mist coat onto the part to be glassed to help keep the cloth from moving around. Spray glue can actually make it much more difficult to apply the cloth and makes it near impossible to smooth out if too much is used.

Spray glue is absolutely unnecessary, adds expense and complicates the job. It's a solution to something that isn't really a problem, but can make it one. I suggest that you don't use spray glue.

Before you start, gather up all the necessary items. Finish sand the parts you plan to glass and then vacuum them if you have a vac with a brush attachment. Otherwise, use a bench brush to wipe off as much dust as possible and then use a tac rag to remove the dust that is left. You can also use compressed air to blow the dust off.

Lay out plenty of newspaper and put on old clothes that you do not mind getting glue all over. A pair of latex gloves will help keep the glue off your hands.

It is difficult to apply cloth with the part flat on the table. The excess cloth needs to hang down so that it will adhere to edges of the component. I use scraps of wood with double stick tape to provide a handle for small or awkward parts. This handle also provides a little space for the cloth to hang.

Keep some alcohol handy for clean up, but again, do not use it to thin the resin.

Let's get started...

Applying fiberglass cloth

As is standard for most finishes, fiberglass is applied in several pieces from bottom to top, rear to front. For example, the fuselage is covered in five pieces. Using a shoulder wing aircraft as an example, I would apply the cloth in this order allowing each to cure long enough to feather the edges before applying the next:

Bottom
One side
Other side
Upper decks (front and rear)

Actually, the order is not critical with glass cloth because all seams will be feathered into each other and they can not lift, so in essence you end up with a seamless finish. Regardless of this, that is the order in which I apply all finishes, so for me it is ingrained.

I learned the hard way that mixing up a large batch of resin thinking I would do several parts at once is a bad idea. The more resin you mix, the faster it cures. In larger quantities, it can get so hot that it melts the plastic mixing cup — I have seen it happen.

I usually do one large component (stabilizer, fuselage side or wing) or a couple smaller components, such as control surfaces, with one batch of epoxy.

By the way, the resin in the pot is not a good indicator of when the epoxy has cured. It will cure faster in the cup than in a thin film. To check if the resin is cured you have to touch the excess cloth on the component. If it is hard, then the resin is cured. If it is rubbery and flexible, leave to cure some more.

As you read through this article, you will see that I have used the term feathering a few times. I was asked to explain what that means. Feathering is simply sanding the edge of a part so it fairs smoothly into the base. In this case it means sanding the edges of the glass cloth after the resin is cured so that there is a seamless transition over the edge of the cloth. You shouldn't feel any bumps or ridges if it is feathered properly.

	Everything needed to apply the glass cloth: The component to be glassed Fiberglass cloth cut about 1" over-size all around Resin Newspaper or drop cloth Scissors Mixing cup and popsicle stick Squeegees Alcohol and paper towels for clean up Latex gloves Holding stick(s) and double-stick tape
	Prepare everything before you get started so that once you begin applying the glass cloth you do not have to stop in the middle to fiddle around with anything. Small items should be attached to something to make them easier to manipulate. Ensure that the part can not come loose while you are working on it by actually tugging on it after it is in place. If the part comes loose in the middle of the job then you will have a really messy situation.
	For medium-size items, a piece of sticky rubber shelf lining attached to a board will help keep it from sliding around. Wipe resin drips from the lining with some alcohol before it sets. The stabilizer was set on this piece and while it did not prevent all movement, it did hold it well enough to prevent problems.
	The shelf lining is stapled to the back of the board.
	Resin should be allowed to begin its chemical reaction before using it. After mixing it according to the manufacturer's directions, stir it thoroughly and let it sit for a few minutes while you cut the cloth. After this time passes, stir again and it is ready to use. This is more than twice as much resin as is actually needed for the part being glassed. It is better to have too much because it is easier to spread out initially and you do not want to run out in the middle of the job.
	Lay the cloth over the component to be covered and gently smooth it. Ensure that the glass hangs over all edges. It is very difficult to reposition the cloth without the weave pulling apart after the resin is applied.
	Pour a liberal amount of resin over the cloth. You want enough so that it is easy to spread, but not so much that it is dripping all over.
	Finishing resin will saturate the cloth immediately because it has low viscosity. It is very easy to spread and is the best resin to use to apply glass cloth.
	Always work from the center towards the edges. Wrinkles do not happen frequently, but when they do they are easily worked out. Trouble spots are normally where the cloth drapes at a corner or where it drapes over end-grain wood. At corners it tends to lift. The fix is to cut up to the corner with a pair of sharp scissors. This will get resin on the blades which should be removed immediately with solvent.
	End grain wood soaks up a lot of resin and sometimes prevents the cloth from staying down. What to do is ensure there is resin in the excess cloth so that it will have more weight to hang down. You should also come back to these spots frequently and squeegee the cloth back down if it has lifted. Sometimes it is a lost cause and the cloth simply will not stay down. The good news is that other than being annoying, it is not a problem. After the resin has cured, put a little CA in the area and rub it down. The cloth will adhere instantly and the problem is fixed.
	After the cloth is fully saturated, use the squeegee to scrape up excess resin. Return the excess to the mixing cup. In this image, the squeegee is being pushed toward you. The resin removed is visible on the bottom edge of the squeegee. Besides preventing unnecessary weight, scraping off excess resin gives a more uniform base. For areas where scraping is impractical, such as the edge of a part, you can blot with paper towels or toilet paper.
	Give the resin plenty of time to cure. Excess cloth must be trimmed and feathered before applying adjacent pieces. It only takes a few minutes to accomplish this using 220 grit sandpaper. Using a block, feather the areas where the cloth will overlap. The feathering does not have to be perfect — just pretty good with no hard, raised edges. Using 30 minute epoxy, you can usually trim and feather after about 4 hours and then glass the opposite side.

Finishing the job

My original statement which I am retracting:

A second coat of resin is often recommended, but it is really the wrong thing to do. This second coat adds excess weight and makes finishing more difficult. Use primer to fill the weave — not more resin.

My Current Stance:

What I have found repeatedly is that I have to apply several coats of primer because as I sand each coat pin holes appear all over. I always assumed it was air bubbles in the primer and didn't have anything to do with the resin.

At this point I'm fairly convinced that the air bubbles are coming from where primer isn't getting under the weave. For my past couple projects I've used a second coat of resin and have been able to apply one normal coat of primer and then touch up a few areas with another coat.

But... the second coat or resin is squeegeed into the cloth well and then squeegeed back off. I'm applying a very thin coating and sanding off the excess.

I've seen glass jobs that look glossy and wavy which is a sure indicator that there is a thick coating of resin above the cloth. That's what we're trying to avoid which is why I recommended against a second coat in the first place.

Moving on...

Do not sand the actual cloth for at least 24 hours. Be patient and allow the resin to fully harden. When all the cloth is applied and fully cured, feather all seams and give the overall component a light sanding to remove dust particles, glass strings, etc. All you are trying to do here is knock off the bumpy crud — a couple strokes across the surfaces is it all it takes. Then stop!

Do not sand into the cloth. The weave is not filled in this process. It is not possible to achieve a glass-smooth finish at this time. The weave must be filled with the next component of the finish — usually primer.

In general, the longer you wait for each coat to dry, the better the finish will be. You may have seen planes at the field where the weave of the glass cloth was clearly visible in the finish. This is due to not allowing time for each coat of the finish to dry thoroughly. As time passes the finish shrinks and sublimates into the substrate.

Primer may feel dry enough to sand after a couple of hours, but it may actually take weeks to dry completely. Realistically nobody wants to wait this long for each coat to dry, but now you know what will happen if you don't.

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About Resins used to Apply Fiberglass Cloth
Fiberglassing a Wing

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