A guide to fiberglassing model aircraft using water based polyurethane
(Fiberglassing without epoxy)

Fiberglassing model aircraft is a well known method of adding considerable durability and strength to a given model airframe. A lot of people would like to apply fiberglass to their model aircraft but are either not willing to part with the cash involved to pay for a good epoxy resin system or simply cannot use epoxy for reasons of health, ie allergic reactions to the various catalysts used. Fortunately you can still use fiberglass for a lot of items without having to use epoxy resin.

Water based polyurethane paints are very cheap, have a good shelf life can come in a wide variety of colours and have very low toxicity, especially compared to epoxy.

So, what is water based polyurethane? It's quite simply what you know as water based paint, sold in all hardware stores and a lot of larger supermarkets and stores. Most modellers opt for clear varnishes and apply their desired colouring over the top, though many tints are available even for clear paint.
Due to the massive variety of brands, it's suggested that you trial various types until you find one that you're happy with. For the purpose of this article we'll be using Cabots Crystal Clear.

Okay, so let's get going and do some fiberglassing!
To be utterly honest, there's not a lot to detail here, using WBPU makes things very easy and can be conducted at a fairly relaxed pace, so first up there's no need to rush. If you've never done fiberglassing before then consider doing a simple piece to start with, rather than risk your entire model aircraft. Perhaps try glass a simple curved canopy or curved block of wood.

Prepare your workpiece
The better you can prepare the surface of what you are glassing, the better the finish will be. Glassing will not magically fill out dents, so if you have any then now is the time to fill them in. Sand back your surface with at least 400 grit before considering to glass it.
 
Secure the workpiece
There's many really good ways to make an utter mess of your hard work, not securing it while you glass is high on that list. If your item is not secured there's a very good chance that it'll fall over and land right into the thick of grit, muck and dust - which will of course totally ruin it.
Cut fiberglass to size
Cut your fiberglass slightly oversize for your job, typically allowing up to 20mm all around is a good idea. If you cut it too close you may find that the cloth will start to pull its threads out (use pinking shears if you have them to assit in preventing thread pulling).
Prime the surface
Because balsa is porous and WBPU is water-based there is a strong absorption factor. Excess absorption of WBPU means excess weight which is something we don't want in nearly all cases. There are several options for priming/sealing the surface;
  • A single coat of thinned dope
  • A light coat of WBPU, allowed to dry to 'touch dry' status (so that it forms a sealing layer)
  • It's also been suggested a light spray of solvent-based paint (clear)
Once we've sealed the surface we will have prevented a major weight gain avenue.
Apply the 'tacking' layer
When glassing with epoxy, a lot of people apply some sort of spray on adhesive to the glass or worksurface to make the glass cloth stick. Instead of using expensive sprays we'll simply apply a thin coat of the WBPU and let it sit for long enough that it becomes sticky (usually about 5~10 minutes). Do not attempt to apply the glass before the WBPU has gone tacky else the glass cloth will simply slide around and cause you frustrations.
Laying of the glass cloth
Now that the tack layer of WBPU is sticky we can apply the glass cloth onto our workpiece and move it into position.
Applying first layer of WBPU to glass

A note to make here - most of the weight in water based polyurethane is in the water so you can actually apply a reasonably liberal amount of WBPU to the glass without worrying too much.

Start painting on your WBPU to the workpiece, starting from the center and working your way out, smoothing down the glass as you go. Now is where you learn why we waited for the initial tack coat to become tacky before applying the glass, for if you had not your glass cloth would now be sliding everywhere as you try to paint on the WBPU.

In the case of the Cabots crystal clear WBPU, when applying it is of a milkshake consistency and a diluted milky colour.

Keep applying the WBPU until the entire work surface is covered, additionally make sure about 5~10mm of the cloth beyond the work surface is also soaked with WBPU (this should happen normally, but do be aware of it), this is so that when the WBPU is dried we can easially cut the excess glass cloth from the job without splinters of glass going everywhere, which is both irritating physically and mentally.

Removing excess WBPU from the workpiece

Although WBPU dries to be fairly light, we should still attempt to remove what excess we can. The process of removing excess WBPU additionally assists in the saturation of the glass cloth due to the pressuring effect when using a squeegee. There are many things you can use as a squeegee, a lot of people like a discarded credit-card.

Again, working from the center of the job, using light pressure, scrape away the excess WBPU, the cloth weave should show up distinctly after the squeegee has passed over. Don't worry about taking too much or little WBPU from the job as we are going to have at least one more coat, possibly two (to cover over the cloth grain).

Watch out for excess paint building up in beads on the sides and undersides of the job. Make sure also that the glass is remaining stuck to the workpiece.

.... wait ....
Depending on the particular brand and variety of water based polyurethane that you used there will be a period of time to wait before the next coat can be applied, in other words, read the instructions on the can of paint, don't just assume. Typically there's about a 3 hour wait - go do something else and don't be near the job, else you'll probably bump into it or stirr up dust which will leave the surface all dirty.
 
Applying the second coat

Now that you've waited the required time for the next coat, you will probably notice that there are a few white "dry" spots on your job, you can try wet them out by dabbing and forcing more paint in that area however generally it's probably just to leave them alone and learn to be more careful on the next job. Quite often they go away with another couple of coats.

Depending on the state of the surface, you may wish to give a light sanding back with 400~800 grit paper. Don't go overboard though else you'll start cutting into the glass cloth and make things worse.

You will require a lot less paint on the second coat due to the glass having already been mostly 'filled'. Again, as with the first coat, start appling the WBPU from the center and working your way out. If you are sparse with your application you will probably find you don't need to squeegee off any excess but do watch out for dribbles or beading on the sides of the job.

 
[optional]Third coat

Depending on the thickness of your weave, you may require more than two coats of WBPU to seal up all the pinholes in the cloth. At this point it's a matter of personal preference and experience.

Trimming and sealing
Trimming your job is best done with a very sharp knife, the sharper the better. Try to cut from the outside to inside, that is, the handle of the blade should be on the glass side of the workpiece as you cut through, this help prevents the glass pulling away from the job.

After you've trimmed away the excess glass you may notice that the edges have gone slightly white, this is normal and does happen frequently, simply run a small dash of WBPU along the edges, this has two purposes;

  • Seals the edge to prevent glass shards breaking off
  • Adheres the edge to the job
All finished.