The next step was to cut out my 1/4" plywood rings. Whether you make your rings from one piece of plywood or two, the process is much the same. You drill holes in the corners, then use a saber saw to cut the inside lines out and round off the outside edges to match the layout lines.
I don't have pictures of the completed rings or of the fiberglass shell preparations (sorry). I think you can visualize how the rings looked anyway. I didn't take pictures of the prep process because we were doing several things at once, and I think the pictures I would have taken would be very confusing.
So here's what need to do to prep your trailer.
First, you need to peel back the insulation and head-liner material that covers your ceiling. If you don't have a vent hole yet, you'll need to cut the hole.
Once you have a hole with the ceiling material peeled back, the next step is to grind or sand the gelcoat off around the top of the opening. Fiberglassing materials, even epoxy, don't stick to gelcoat materials very well, so exposing a roughly 2" (50mm) band of raw fiberglass around the opening allows you to attach the vent ring in a way that prevents leaks
Which is kind of the point of all of this.
A good tool to use for doing this is a grinder with a 120 grit flap disk, available for $25 or less from Harbor Freight when they're on sale. A grinder removes the gelcoat almost effortlessly. (Which is why I don't suggest using a flap disk with coarser abrasive sheets; they can grind through gelcoat and through your shell much too easily.
I don't have a good picture showing how far I ground the fiberglass back from outside the trailer, but I do from inside and underneath. This picture shows what the opening looks like after my bottom-side ring was attached. You can see the light
coming through the fiberglass from above where the gelcoat has been removed, and I think it shows the margins you need around the mounting ring pretty well.
A couple things worth noting here. One is there's a notch cut in the forward part of my inside ring where the fan wiring can get through. The other two are, first, you can see where we patched the roof toward the back of our fan opening, because we wanted to move the vent forward several inches, and the fiberglassed-in roof supports. I'll talk about the latter two things another time, but do make sure you have a gap in your bottom ring where the wires can pull through.
Anyway, you've seen the after picture for the bottom ring. Getting the bottom ring into place isn't too hard to do. The easy way is to spritz the top side of the ring and underside of the trailer shell with water, then apply urethane glue to the upper side of your bottom ring, line it up with the opening, and clamp it into place. (Spritzing the surfaces with water might sound counter-productive, but urethane glue needs the moisture to set and cure.) Make sure to use lots of clamps to assure tight bonding between shell and wood frame, and trim off any excess urethane that oozes out during the curing process after the glue has fully set with a sharp utility knife.
Mounting the outside ring is much the same, but, instead of urethane glue, you'll want to use a ring of thick, resin-saturated fiberglass mat or a thick layer of Bondo Glass filler to glue the ring into place. The glass mat or bondo helps make sure you have full and complete contact between the ring and the fiberglass of the outer shell.
The advantage of using the resin-saturated mat is it gives you about 15 minutes working time to position and clamp your ring into place. The disadvantage of resin-impregnated mat is it's drippy and smelly, and working with it overhead while you clamp it into place is messy. Wear a hat or shower cap while you work with it.
The advantage of Bondo Glass is it does an even better job of filling any voids between ring and shell and stays in-place on the ring as you move it from workbench to trailer roof, but Bondo sets in about five minutes, so you have to butter it onto the ring, get the ring up through the hole and lined up, then firmly clamp it into place all in the space of five minutes. That's not much time to get everything right.
Once the upper ring was in place, I sanded the inside edges of the opening smooth, then built a little 45-degree ramp of Bondo Filler all the way around the outside edge of the top ring. Nothing special about doing this; it doesn't have to be pretty, just fill the sides in and create a ramp. I built the edge up using two mixings of Bondo, and sanded any really rough spots smooth before the next step. (No pictures, sorry.)
Once the Bondo ramp had cured (about 15 minutes), I fiberglassed the top side of the ring to the fiberglass shell of the trailer shell using four 2-1/2" (63mm) strips of lightweight fiberglass mat.
Here's a picture showing the angle of the Bondo ramp I built with the fiberglass over it.
The final step before painting
-- which will be a while, because we have other work to do around our trailer -- is to use Bondo filler and a power sander to smooth out the outer shell and ring ramp so it looks pretty.