Repair of a Crack in Fiberglass
Cracked fiberglass panels should be repaired before the crack worsens. A crack can continue to spread and also cause leaks through the fiberglass panel. Gelcoat is intended to be a protective barrier to keep moisture from the underlying surface.
Cracks also are ugly. They detract from the good looks of a fiberglass panel.
Fortunately these can be repaired. A short demonstration of the basics from BestBathUSA on Youtube.
The actual repair is more involved, but it is possible to do it yourself. The correct tools and materials are required, but are widely available.
Making a Small Mold
There are many methods and materials that can be used for making a fiberglass composite mold.
I found a video from Eastbay Composites that demonstrates a method for making small molds in a very quick an inexpensive fashion.
As you see from watching this, the basic construction materials are tooling gelcoat and inexpensive bondo for the support structure.
This mold construction technique certainly has drawbacks, but also has several advantages. None of the materials required spray equipment, which is messy. Brushes are inexpensive and easily disposed. A disadvantage of this method is that surface finish might suffer from a lack of consistency on the coating thicknesses for the gelcoat and the release agents.
Using bondo as the support structure is quick and easy compared to glass and resin, but can cause problems as well. It is more likely to crack, and can warpage issues as it cures and may be hotter in some areas than others. Bondo may have some difficulty maintaining dimensional tolerance as it shrinks during cure. Industrial resins used to create molds have minimal shrinkage in their chemistry and are placed on slower to minimize heat from the chemical reaction.
For quick, inexpensive and easy parts, this method from Eastbay Composites may work well for you. What has not been discussed yet is that the mold model can be the most difficult part. Off the shelf items are easy, but custom ones may be difficult. Creating a shape or surface is time consuming and tenuous. After that is complete, you can make your mold and final production parts!
Cleaning Fiberglass Boats and RVs
Many people have fiberglass items that require cleaning as part of regular appearance maintenance. Figuring out what materials and cleaning agents to use can be difficult.
When people inquire with me, I point them in two different directions.
One direction is to consider household cleaners, especially those for bathrooms. Fiberglass bathtubs are exposed to many chemicals and dirty water, etc. They are prone to staining due to the nature of their application and exposures. Many fine products have been developed to enable homeowners to clean and maintain their bathtubs in a reliable fashion. These products are easily found in grocery stores.
Another direction that can identify good methods to improve the finish of fiberglass gelcoated structures is the materials and methods of the Autobody industry. Automotive paint shares many properties similar to gelcoat. Dirt, grime, and scratches can be removed from gelcoat in many of the same ways as painted surfaces. The same sandpapers, sanding tools, buffing compounds, and buffers can be used to improve gelcoated surfaces.
Working with surface coatings such as paint and gelcoat can produce rewarding results. One caution when sanding, polishing, and buffing these surfaces is to maintain caution regarding excessive removal of these coatings. Sanding and buffing removes microscopic layers of material.
Exposing layers below the outermost ones can produce disastrous results which may require major expense in repair. New layers of paint or gelcoat will need to be applied to restore the integrity and appearance of the original coating.
Expensive specialty cleaners for boats and RVs might seem necessary, but I am betting that less expensive alternatives from the bathroom and body shop will do just as good of a job.
Gelcoat Spraying for Repair
Gelcoating a repair area is challenging, but can be accomplished. It requires the proper equipment and materials along with some good techniques in order to achieve a nice finish. Do-overs are possible if the results are not as intended.
Gelcoat can be sprayed with a conventional paint spray gun. I recommend purchasing an inexpensive gravity feed, though suction feed will work as well. The gun must be equipped with a rather large spray tip due to the higher viscosity of the gelcoat.
The surface to be sprayed must be prepared correctly. This includes sanding with 180 to 320 grit sandpaper and removing all contamination. The surface must be uniform and to the intended shape, with any transistions between surfaces smooth and consistant.
Gelcoat in a mixing cup needs to be thinned to lower the viscosity to a sprayable level. This can be accomplished with a repair additive, MEK solvent, clear gelcoat, or acetone. Some of these materials are recommended more than others, and acetone especially is one to try and avoid. Before spraying, the catalyst needs to be added at the recommended level.
Spraying standard gelcoat onto an exterior surface will normally leave the surface tacky and difficult to sand. This can be avoided by adding some liquid paraffin wax (about the same amount as the catalyst) or by immediately following the sprayjob with a layer of water-based Partall Film #10. Basically the top surface of the freshly-sprayed gelcoat needs to cure without being in contact with the air. The paraffin wax rises to the surface to accomplish this, or the Partall Film lays on the surface to accomplish this.
The spraying of the gelcoat needs to be accomplished quickly once the catalyst is added. The gel time is running after the catalyst is added. This includes cleaning out the gun. This will be easily learned if you are spraying along and the gun stops spraying. After opening the lid you see a semi-solid mass. Now you have to hustle to get the gun apart and the gelcoat chunks cleaned out before cure.
Acetone is the best way to clean the gun out well. The needle, seat, nozzle, and cup all need to have all residue cleaned and flushed. Any remaining gelcoat will cure inside the gun and may ruin it for future use.
Hopefully you have a good cure and a wonderful result!
Alligatoring Causes
One of the challenges of using Gelcoat is the potential for the defect called alligatoring. This is described as a wrinkled surface on the completed part after it is removed from the mold, and results in extensive post-mold repairs to the cosmetic surface of the part.
There are four scenarios that may cause this alligatoring to occur.
Applying the gelcoat film too thin is the primary cause. This thin layer of gelcoat will lose a larger proportion of monomer to evaporation than thicker layers of gelcoat. The crosslinking process will deviate from the manufacturer’s intended formulation. This undercured gelcoat is then attacked by the laminating resin which wrinkles the gelcoat layer. Applying the manufacturer-specified mil thickness of gelcoat will prevent this condition from happening.
Another scenario of alligatoring is caused by laminating on undercured gelcoat that is the correct mil thickness. This undercure may be caused by: insufficient cure time, insufficient cure temperature, initiator problems, and compressed air contamination. Closely following the manufacturer’s specifications with regards to shop conditions, initiators, and proper equipment will minimize these problems.
The spraying process is another cause of alligatoring. Keeping a wet edge is important. This means that the new gelcoat is applied over gelcoat that is still wet, which allows for all of the gelcoat to cure at the same time. Fresh gelcoat sprayed over cured gelcoat will attack itself and can lead to alligatoring problems.
A fourth cause of alligatoring can be the result of exposure to a solvent such as MEKP or acetone before the laminate is applied. The most common cause for this is an equipment leak during laminating operations. Solvent rags placed on the gelcoated surface may also be a culprit.
These above are a few of the causes of gelcoat alligatoring. Once these conditions are under control, few problems will present themselves.
Gelcoat Application
Making a traditional fiberglass part is accomplished by working from the outside surface into the part. A clean and waxed mold is placed in a clean room where contamination will not interfere. Then gelcoat is sprayed on as the first layer, and followed by the rest of the laminate. The gelcoat application process is much more art than science.
The objective of gelcoat is to create a uniform thickness across the part, and have it be 18 to 25 mils thick when wet. As it cures, this thickness is reduced when some of the chemicals evaporate off. Spraying gelcoat onto a mold returns the best quality finish, but it may also be brushed in areas that are difficult to spray. Achieving a uniform thickness is difficult at intersecting corners, deep narrow areas, and difficult-to-reach sections of the mold.
The gelcoat is typically applied in three passes and allowed to “gas off” in between coats. This allows for some of the chemical evaporation to start, and can lead to problems if not done correctly. These passes are also usually bi-directional, where the first and third pass are in one direction and the second is in another, again to help achieve uniformity.
Areas of excessive gelcoat thickness can lead to cracking and surface finish problems. Areas of insufficient gelcoat thickness can “alligator” which is a surface flaw requiring extensive repair.
Gelcoat Application is a tricky part of the process and the manufacturer of the application equipment and the raw material is the best resource for best practices. Experience is the best formula for great results.
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