Many of our RVs and boats have a lot of fiberglass components. Fiberglass is quite durable, fairly resilient, and not too difficult to repair. Interestingly enough, fiberglass will often resist impacts that would leave dents in metal components, sparing us some of what could be costly cosmetic repairs. When fiberglass is damaged though, it takes special treatment to effect proper repairs. You can get it done a professional body shops but if you are fairly handy you may be able to make many repairs yourself, unless you have an expensive vehicle that warrants the cost of professional repair. Some big components (like front or rear caps on RVs) are sometimes damaged beyond repair and may have to be completely replaced but cracks and holes in the fiberglass paneling of RVs or fiberglass decks and hulls of boats can often lend themselves to DIY repairs.
The plastic components of many OHVs don't lend themselves well to being repaired. Mostly they should be replaced and are usually fairly easy to replace and not TOO expensive. Side-by-sides however often have larger, more expensive components you might want to try to repair but they too usually have to be replaced to get a like-new appearance.
Fiberglass resin is also used to repair damage to metal body panels on cars and trucks. It is used about the same way as you would use it on fiberglass. Before Bondo and similar fiberglass repair materials, body repairmen used lead to fill and smooth welded repairs. Lead makes a good fairly permanent repair but it requires a lot more time, equipment, and skill than fiberglass resin and presents a certain health risk so it has fallen out of normal use today.
Bondo is probably the best known of the fiberglass repair products. It is used to repair dents in metal body parts on vehicles as well as on fiberglass. Bondo is reasonably priced and fairly easy to work with. It can be used by itself to fill small holes or with fiberglass cloth for larger repairs. Ambient temperatures need to be above 40F for it to work properly. When you buy Bondo it usually comes in a can with plastic top that covers a tube of hardener. Pay close attention to the instructions for how much hardener to use and how to mix it. As I recall a general rule is to use about a 1" strip of hardener squeezed from the tube for about a golf-ball sized glob of filler. If you don't use enough hardener, it will be slow to setup or may never set up; if you use too much, it will harden too quickly, before you can spread it properly. Don't mix up more material than you can use in just a few minutes or it will harden before you can use it.
Repairs on RVs and above water repairs on boats can usually be done in situ, where ever the unit is normally kept. Repairs below the waterline will usually require hauling the boat out of the water before repairs can be attempted. Because of the criticality of repairs below the waterline you will want to take extra care or have them done by a professional. A mistake in an above water repair could be embarrassing, unsightly, or even costly. A poor repair below the waterline could end up sinking your boat!
Doing fiberglass repairs is fairly safe but there are some requirements for personal protective equipment and procedures. You should always wear a face mask or respirator when working with fiberglass resin and when sanding cured repairs. Latex or rubber gloves are advised when mixing and using resin and when painting to avoid contaminating and injuring your skin.
Always check for structural damage before beginning repairs. Many times cosmetic damage is he result of or part of structural damage beneath the surface. Fiberglass boats usually have a wooden core between two layers of fiberglass that makes up the hull and bulkheads. This wooden core can be compromised if water leaks in anywhere around hardware that penetrates the fiberglass, weakening and causing it to fail. Repairing or replace rotted core is a difficult and expensive process but simply patching the fiberglass over a damaged core is a sure recipe for failure! Impact damage that crushes fiberglass on boat and RVs can often damage structural members inside. RVs usually have wooden (or sometimes metal) framing, kind of like a house. If any of these framing member have been damaged you will need to open things enough to repair the structural damage before repairing the skin.
As with most tasks, preparation is a large part of making successful repairs. When repairing fiberglass you may need to cut away damaged portions so they don't weaken the repair. Broken, cracked, or otherwise damaged material should be removed so it doesn't weaken the repair. Often you will want to drill a small hole at the end of crack to prevent it from continuing to extend into previously undamaged material. It is also essential to thoroughly clean the surfaces around the repair where the Bondo will be applied to blend the patch into the surrounding surface. Sometimes you may need a backing plate inside the component to be repaired to give a solid base upon which to construct the repair. Backing plates may be temporary (long enough to make the repair) or permanent (part of the repair). Check any structural components within the repair and fix any damage before repairing the covering. For large holes spread a layer of mixed filler, then cover it with a piece of fiberglass cloth cut to fit over the hole. Then cover the fiberglass with more filler, tapering it toward the edges. For deep holes or dents you may have to apply multiple layers of patching. Really large holes may require the addition of patch panels. Repeat as necessary to make it level with the surrounding area. Eventually you will sand the cured repair until it blends smoothly into the surrounding surface. I usually do initial shaping using a body file, then #60 sandpaper followed by successively finer grades of sandpaper down to at least #200 or #220 before applying primer. If you are working with a deep dent it may take several layers of resin and fiberglass to build it back up to where it needs to be. Be sure the let each layer dry according to the manufacturer's instructions but don't let them over-dry. You want multiple layers to bond seamlessly with each other. While it might be tempting to just fill up the hole all at one time that can prevent the bottom part from curing properly, leaving it somewhat soft and weakening the repair.
Once the application has cured (Bondo is usually dry in 10-15 minutes) you will need to file or sand it to remove any imperfections and blend the repair smoothly into the surrounding surfaces. For best results you should then apply a finishing paste. It is similar to the material used to build up the patch but is made of finer material to produce a smoother final surface. Allow the finishing paste to dry and cure according the manufacturer's instructions, then sand it again until everything is really smooth. The first step is usually using a body file or another coarse file to knock down any high spots. Once you have things generally level, initial rough sanding may be done with coarse (#60 or #80) sandpaper. Then used increasingly fine sandpaper (#100, #120, # 150, #220, #200 etc) until it is completely smooth. Final sanding is often done with #400 or even #600 wet and dry paper. If you are working with a long stand fiber repair product like Evercoat Kitty Hair and even for heavy Bondo repairs, you will most likely need to initially shape the repair with a body file or wood rasps and files to knock down the high spots before sanding. I have found it typically takes 2 or 3 applications of Bondo and/or finishing paste to properly fill holes and return the surface to its proper shape. Be sure to plan for that and allow enough time for multiple applications, with plenty of sanding and shaping between coats. If you can see or especially if you can feel any edge or discrepancy in the surface of the repair it must be corrected before it is painted or it is going to look really bad.
Next apply a good coat of primer over the repaired area. This is a step that is often ignored by amateurs, usually yielding poor results. Primer is designed to bond tightly to the surface and to bond tightly to the finish layer of paint. Top coats are designed to give a good final appearance and seal the surface but they aren't designed to bond tightly to bare surfaces. Let the primer dry as needed. To achieve the smoothest, shiniest results you should sand the primer with #400 wet and dry sandpaper before applying the top coat. Read the directions on the primer can to see if you need more than one coat. For best results, sand between each coat. Some finish coats may claim they can be used without primer and those might work. However, you will usually get more permanent and better looking results using a real primer first. If you still see or feel any edges after priming, you may have to go back and redo some of the repair work again to make it smooth so it looks good. A good repair will be invisible once the final coat is on.
Apply the topcoat according to the manufacturer's instructions. You may want to apply multiple coats for maximum beauty and performance, possibly sanding with #400 or #600 wet and dry sandpaper between coats. Do not sand after the final top coat!
Inspect your repairs after the final topcoat has dried. Sometimes you will find minor flaws in your repair that don't show up until then. Tiny differences in texture might not show up until after the finish coat as cured. If you do spot them, let the paint cure completely, then sand the offending spots down again until the flaws disappear and refinish the area with primer and topcoat again. Sometimes it takes several interations to make repairs invisible but the results are well worth the extra effort. It recently took at least 4 passes for me to get satisfactory results after repairing a few simple chips in the bow of my sailboat. Some of those passes required going all the way back to redoing the Bondo!
I recently came across another product that claims to be stronger and more waterproof than Bondo. It also contains long strands of fiberglass within the resin so it doesn't require fiberglass cloth for some of the smaller applications. It is called Evercoat Kitty Hair. It is a little more expensive than Bondo but not needing fiberglass cloth for smaller repairs and the additional strength and waterproofing may make it well worth the extra investment. It is a lot faster than cutting fiberglass cloth to fill large holes (up to about the size of an egg -- holes bigger than that require additional repair with appropriate sheeting material and fiberglass cloth).
I have had a chance to try out Kitty Hair and it did a wonderful job of filling an egg-sized dent in the bow of my fiberglass sailboat. I did discover that it is a little harder to smooth out than ordinary Bondo and I ended up using Bondo as the final coat before finishing paste, primer and paint. Kitty Hair certainly worked well to build up a sturdy filler, much easier than repeatedly cutting graduated pieces of fiberglass cloth to fill in the hole. I will still used ordinary Bondo for shallow scratches where the long strand fibers in Kitty Hair would be overkill and would be difficult to smooth out. As mentioned above, I used rasps and files for the initial shaping followed by #60, #80, #100, #150, and #220 sanding before applying the primer. The best file for this is called a body file and is specifically designed for working fiberglass fillers. Body files are flat files with wide curved grooves that smooth scrape away high spots on fiberglass filler. However, lacking a body file, you can usually get the job done with ordinary wood rasps and files if you are careful how you use them! Be especially careful if you use a wood rasp. The rough, sharp teeth can make deep gouges in the filler and you will have to fill them in and start over!
There are a lot of good videos on Youtube for both vehicle and vessel fiberglass repairs. It would be well worth your time to view a few of these before embarking on your first attempt. With a little patience and practice you can make a lot of repairs yourself, saving both time and money.
Many fiberglass boats (and RVs) have a gelcoat finish. Repairing the gelcoat takes special gelcoat compound along with rather tedious preparation and application. Sometimes you can use Topside paint to cover the repair if you can match the color closely to the gelcoat. It is difficult if not impossible to blend the paint into gelcoat for a seamless repair. For that you need matching gelcoat. If you opt to paint it instead, it might be best to mask off an appropriate section do be repainted. If you still don't like the way it looks you might try applying some kind of decorative design to disguise the repair rather than trying to blend it it. The design might involve vinyl decals or paint. I used the masking technique when I had to repair a ding in the bow of our sailboat, painting the whole bow from deck to waterline for about 8". The painted area is detectable up close but does not stand out and is WAY better than the original damage! For best results get a gelcoat repair kit to match the finish on your vessel or vehicle. In the long run it will be well worth the extra cost and extra effort. Here is a Youtube video on gelcoat repair.
Rainy days? Can't work outside? Might still be able to get some things done. A lot of fine sanding, like between coats of paint, are done using #400 wet-and-dry sandpaper and sanding wet. So even if the surface is wet you can sand it. You probably don't want to be standing out in a pouring rain. That would be uncomfortable and it might be too much water even for wet sanding, but you might still be able to make some progress on a project without having to wait for another sunny day. I did indeed take advantage of a couple of rainy days to do the wet sanding between top coats when I repaired the bow of my sailboat. Having to work on rainy days is pretty much a given here in Oregon! My dad used to say there were only 3 seasons in Oregon: Spring, Summer, and Rain -- and it rained in the Spring and the Summer!
Patch it up!
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