Whether you are getting ready to fix your bathtub, boat, Truck Cap or other Fiberglass item that has seen some abuse you are probably going to need to replace its outer gel coat in the process.
Many fiberglass parts are made in a mold and to produce a quality smooth surface the technician will coat the mold with a gel coat and then apply fiberglass over it on the inside of the part.
This is the process used for many automotive and boat products but you can also see it on any fiberglass part.
Once the item is removed from the mold the gel coat is sanded and in some cases it is painted but in the case of boats and many other items the gel coat is often color tinted and used for the final color.
Gel Coat Colors
Gel Coat is kinda like an epoxy paint in that it requires the base liquid and a hardener to cure.
You can purchase gel coat in clear or a solid color that is pre-mixed. Various special colors of white make matching the gel coat to your boat sometimes difficult.
If you have a custom item then the gel coat may have a mixed color.
You can purchase your gel coat with a standard set of mixing colors.
Usually you will only use a small amount of a few tints to get the right color match.
Mixing for color tint is always performed before the hardener is added.
Preparing the Surface
When you are working with older fiberglass parts that have had long term exposure to the weather because of a crack in the gel coat you will find that the water and maybe freezing and thawing of ice over the winter has caused the fiberglass to breakdown.
It is important that you make all your fiberglass repairs on the inside or replace the section if needed.
Just like your paint job on your car you want to leave it for last and although you would never paint your car if you were going to replace a fender its probably not a good idea to try to fit a new engine in or do heavy suspension work modifications. So even if you are working on a boat hull you should complete everything else first including fitting of any structural pieces inside before you begin your gel coat work.
In other howtos we will cover fiberglass work in detail but in the case of restoring a complex part before you begin work you may want to make a partial mold which you can attach to the area on the outside while you are building up your fiberglass on the inside.
Trying to match complex compound curves of a hull bottom could be hard work so think about making a mold then removing it temporarily so you can cut out the bad section. Then reattach the mold before you begin laying up your fiberglass from the inside.
A plaster of paris mold will do well in this situation to help you retain the curve shapes for the bulk of the work.
Applying the Gel Coat
Once you have decided on a color and are ready to get started you should begin by cleaning the area with acetone. Use gloves while you work with acetone because it will get into your skin and cause you irritation or other problems. READ THE WARNINGS ON THE CAN.
Acetone will also displace any water in the fiberglass.
Sand the final fiberglass surface with a 100 grit sand paper.
Make sure you have removed all of the cracks down to the fiberglass and a 1/4 inch bevel on either side of the crack.
Mix your gel coat as recommended on the can and add your color if required.
Apply the Gel Coat so that it stands proud of the surrounding surface by about an 1/8th of an inch. This will allow for curing and sanding.
Final Sanding of the Gel Coat
Once the gel coat has cured you will want to begin sanding with a 400 grit wet sandpaper using water.
Sand the area level using a sanding block if needed.
Finish sanding by using a 600, 800 then a 1000 grit sand paper that will reduce almost all of the scratches to a haze.
Once you are happy with the surface you can polish with a buffer or hand polish with automotive buffing compound.