We all realize there is more than one way to do things and this thread is a history of how I am doing things on my 1971 Campster renovation project. This is not my first fiberglass RV renovation.
Once upon a time there was a leaky and corroded, ancient electrical
and now there is a hole that needs to be filled
I saved some patching material when I took out the old kitchen counter
Just tape it in place and trace the outline of the hole onto the patching
I cut around the outline using a bimetal blade on a bandsaw
After a little test fitting and minor adjustments it fit just right.
Next on the outside of the trailer I sanded a bevel on the edges of the opening and I also sanded a bevel on the edges of the patch. This helps to widen the surface area and the depth of the epoxy adhesive which helps to prevent cracks developing along those edges.
I used duct tape on the inside to hold the patch in place making sure the tape spanned over all the gaps. That tape is now acting as a backer to keep the filler materials from pushing all the way on through the gap as I work. The outside surface of the patch should be level to the surface of the exterior wall or slightly higher. The wall was not perfectly flat but that did not matter as long as the patch was not below that surface level. Sanding to level is better than adding filler to bring it up level. I start securing the patch in place using an Epoxy Putty stick for the adhesive/gap filler. Begin in two corners, let it harden then do the other two corners. That way it is fixed in position but if you did need to adjust the height you only have a little bit of epoxy to remove to break the patch free.
Now finish filling all around the edges of the patch with the epoxy putty. It will stick to itself so you can add more, sand some after it cures and add more again if needed. Then do a final surface level sanding. I use a random orbital sander for this work. Coarser grit first then 220 for the final.
My patch might look not very even in this photo but when you run your hand over the surface and look at it in a sideways glancing light
you will find no difference in the surface of the materials. It is now ready for cleaning, priming and paint
You can see how the sander took off some the the surface on the patch where it had been higher than the surface of the wall.
If this was a patch for a much larger area I would have added a layer of cloth over the outside of the wall to strengthen the joined edges. But that is not necessary for a hole this size that is not going to have a lot of force or stress applied to it. Filling in a window opening would have required that extra layer of fiberglass.
There will be fiberglass cloth and resin applied over patch on the inside of the wall to reinforce it. A couple of layers of cloth with resin will be enough. I don't need to show you a standard fiberglass cloth and resin patch on the inside of a rough fiberglass interior wall as there is nothing special about doing that.
I would never use Bondo for leveling fiberglass as it is a hygrosopic material meaning that it absorbs moisture even after it cures. I you do need to use a surface filler that is easy to sand you can mix micro balloons into thick viscoscity epoxy resin. That is how it is done in boat yards. The epoxy makes a strong bond to the fiberglass gel coat.
I use the epoxy putty stick material to fill in all small holes, dings and chips in the gel coat. It is quick to cure, easy to mix and apply and it bonds well to the gel coat and any uncoated fiberglass. The epoxy putty stick I am using here is PC Lumber
, it does not matter that the label say lumber it also works great on fiberglass. I like that the color is a light
beige and also this formula has a very smooth, creamy texture that sands out nicely.
I have time tested this patching method, I used it 5 years ago on renovating my fiberglass Sunrader motorhome and there has been no failure of the work.