The fairing process can be both, the most rewarding and the most frustrating part of boat building.
Rewarding because this is where your build can become a showpiece (if you want it to.) However, if you want a showpiece, take care to try to build a showpiece every step of the way. Don’t build a workboat and expect the fairing process to turn it into a showpiece.
Fairing can be frustrating because when you don’t approach it correctly. It can take a while and there will be times where you feel you are not making progress equal to the work.
We will try to explain how to avoid the ‘pitfalls’ of fairing.
Understand that there is a great difference between smooth and fair. A boat can be very smooth but not fair and when paint is applied it will NOT look good.
If you wet the surface and the reflections of objects in the wet surface are not jagged or wavy, most likely the surface is fair. Fairing, obviously, is the process of bringing a surface from unfair to fair.
How is it accomplished? This method will attempt to explain the process.
Required for Fairing
- Fairing materials
- Tools for fairing
In our shop, we use products from Alexseal to perform the fairing process. While other companies make fairing products, those from Alexseal are the ones we are familiar with and so we will refer to those in this document.
They are listed below in the order they are typically used:
- 302 Super Build Primer (Base, Converter and Reducer)
This material creates the perfect bond between the fiberglass and the rest of the fairing system. We prefer to spray but it can be rolled on.
- 202 Trowelable Fairing Compound (Base and Converter)
To achieve mixture ratio of one-to-one, we use ‘custom’ tools we made from 4” PVC pipe. They are simply rings to which we mounted handles. We have one for the base and one for the converter. These rings are of equal height and ensure that we always mix equal parts of base and converter.
202 comes in two ‘flavors’: Fast and Slow. Fast, when mixed, is red in color and Slow, when mixed, is grey in color. Slow, really needs to cure overnight before you can sand it. Fast, if applied in the morning, can usually be sanded the same day.
- 328 SprayFair (Base and Converter)
328 is dark brow when you first apply it. As you sand it, it becomes a lighter brown. This is VERY helful. Any low spots are quickly identified by the darker color due to the sander passing over but not touching.
This material is a bit softer and easier to sand than 202. It is also not recommended by the manufacturer for use below the waterline. Some builders do use it below the waterline. Our opinion is that if the manufacturer says “Don’t” then we don’t. Simple as that.
This articleis expected to be used as a high-level guide to the fairing process and materials. Alexseal has published documents that go into very great detail on the use of their products. They do a nice job of describing, for example, the appropriate range of sandpaper grits to use to properly prepare a surface prior to the application of a specific product. Please take the time to familiarize yourself with all of the great information they have to offer on their website.
Tools for Fairing
These include tools to:
- Apply the fairing materials
- Sand the fairing materials
- Test the surface for fairness
1. Tools to apply the fairing materials
a. Troweling / spreading tools
i. Scooping, Mixing tools
We scoop the 202 Trowelable Fairing Compound (Part A Base and Part B Converter) out of their respective buckets with dedicated 3 inch metal putty knives.
You must be careful to not accidentally introduce any of Base into the Converter bucket and vice versa. You don’t want any possibility to initiate the chemical reaction of this expensive material in the buckets.
Mix the 202 Trowelable Fairing Compound (Part A Base and Part B Converter) on a dedicated board reserved for this purpose. To mix, we use two more three inch metal putty knives using a “fold and knead” process, again and again, until no more streaks of Part A and B are detectable.
Before you can effectively spread the 202 Trowelable Fairing Compound, you have to apply it to the surface of the boat. To do this, we typically use a 12 inch drywall knife. This tool is not used to achieve a final finish, it is only used to prepare the 202 for the “pull”. We call it that because troweling is almost always a pulling motion.
ii. “Pulling” tools.
All the tools we use are made from aluminum and vary in length, width and flexibility.
Some are quite long and not flexible (angle). We use these on flat surfaces such as transoms. Some are long and a little flexible – used by multiple people on flatter areas of the boat – such as the sides close to the transom. Other tools need to be shorter and more flexible and are made from flat stock. The best way to determine the appropriate tool for the “pull” is to simulate the application of the 202 material with dry tools. That way, you can also determine the best direction for the pull.
Note: You should file the sharp corners of your tools into a slight radius prior to first use to avoid digging a sharp corner into your boat.
b. Spraying tools
i. Alexseal 302 SuperBuild Primer
As mentioned, the material can be sprayed or rolled on. We prefer to spray. A gravity feed spray gun with a primer tip will do the job.
One more thing: All of our spray guns are equipped with PPS adapters from 3M. Click here to learn about the PPS. Once you try it, you will NEVER go back.
ii. Alexseal 328 SprayFair
328 is a very thick material to spray. We use a pressure-pot spray gun with a 2.5 tip. A gravity feed gun will not work. Click here for the gun we use. (If you don’t feel like investing in this gun, simply roll the material on.)
c. Sand the fairing materials
Longboards are the primary tools used to sand the boat fair. We standardized on two different widths and many different lengths and flexibilities. With only two widths, it is easier to manage the supply of the necessary sandpaper. The two widths we use are 3 inch and 4-1/2 inch.
You can buy longboards or make them. We choose to make them.
All of our longboards started out life as strips of 1/2 inch thick Baltic Birch plywood. Why Baltic Birch? Baltic Birch plywood is usually made from even number of layers of birch, which is a hardwood. It is then glued with an exterior grade adhesive. This creates a very strong and balanced panel, which in turn, makes a very straight and strong base for the longboard. Now we have to decide, based on where on the boat we intend to use it, how long, wide, and how flexible to make the longboard.
If we want the board more flexible we simply plane the strip down until the desired flexibility is achieved. If we want it completely stiff, we glue and screw a full length handle to it. This basically locks it to perfectly straight and completely stiff.
We make the handles by milling pine 2x4s to the cross-section in the illustration below and then cut them to the proper length for the longboard; either full length for a completely straight longboard or into six inch lengths that we glue and screw to the boards in the quantity and spacing appropriate for each longboard.
Attaching Sandpaper to the longboards . We prefer to buy hook and loop sandpaper in rolls. The paper has the “loop” portion glued to the back of the paper and so we need to apply the “hook” to the longboard. We buy it by the yard from McMaster-Carr (SKU: 94975K42). Click here to view it online. We cut it to fit and staple it onto the board.
We also use a variety of rubber sanding blocks that are sold under the brand name Dura-Block. These use PSA (Pressure Sensitive Adhesive) sandpaper in 3 inch widths.
Technically, the blocks are 2-3/4 inches (not 3 inches) but as we said before, we try to minimize the types, grits, and widths of sandpaper we stock as it is expensive.
d. Inspection tools
We turn off the lights in our shop and use a very bright LED flashlight. We shine the light at a very shallow angle to the surface (almost parallel) to very quickly reveal any areas requiring more work. One thing to remember, the light will reveal EVERYTHING; much more even than the human eye can detect on a dark and well-painted, glossy surface. So, don’t get overly alarmed at the apparent lack of surface quality the first time you do it.