Stitch and Glue 101 - Fairing & Sanding
Before fairing, check that all surfaces inside and outside are coated with epoxy resin. Fairing is not absolutely required. If you are satisfied with a work boat finish, a quick sanding and paint will be sufficient but most builders will fair to a certain degree for a better finish. Fairing is a two step process. The first step is fairing for a smooth, fair hull. During that stage we correct the uneveness of parts of the hull. We grind down high spots like the edge of the tape and fill low spots, along the edge of the tape. During that stage, we will use coarse grit sand paper: from 40 to 80 grit. Gaps and low spots will be filled with a fairing putty, ideally QuickFair.
Fairing Putty, Comments and Recommendations:
You can make your own fairing compound from epoxy resin mixed with a filler or work with a premixed putty like QuickFair. The fairing putty must be made with epoxy resin. Auto body shop putties like Bondo are unacceptable. Those polyester based putties do not bond properly to epoxy, they will flake off.
Prepare your own putty using a filler made of a mix of micro balloons and colloidal silica. Silica cures to a very hard finish but is required in the mix for thixotropy. Without it, the putty would run or sag along vertical surfaces. Do not waste time with weird putty recipes, fillers are cheap, use what works.
Recipes to make your own fairing putty are listed at our web sites.
Premixed putties have advantages. Not only are they easier to use but they cure much faster than home made putties and are much easier to sand. If you make your own fairing putty, you may have to wait between 24 and 72 hours for a complete cure. You can't sand an uncured putty, it will clog the sanding disk. QuickFair will cure and be ready to sand in a few hours.
Check fairness before applying putty. This is done by running a straight edge on the bottom or bending a batten along the sides.
This picture is not a good example: normally we use an aluminum beam, 2" on edge, 8 to 10' long and run it at a 20 degree angle with the keel.
The picture above shows QuickFair applied along the taped seam.
Thanks to our "poor man's peel ply", there is very little to fill.
We use wide flexible plastic putty knives to apply the putty.
How to Mix QuickFair:
If you make your own putty, mix your resin as usual and add fillers to the desired consistency: make a firm putty or a slurry depending on the job.
To mix QuickFair is faster and easier.
QuickFair is made of two components, a dark and a white paste.
You must use two volumes of the dark paste for one volume of the white one.
We devised an easy way to measure.
Take two wood strips of the same width (paint stirrers) and mark them every inch.
One inch of withe paste will be mixed with two inches of dark paste.
We have measured many batches that way and they always cured perfectly.
Mix until you obtain an homogeneous color and apply.
The sanding is usually done with a DA: a dual action random orbital sander. This is the tool that you will probably use the most during the building of our boats.
After a few passes, your hull should be fair and ready for the primer.
The goal of fairing with putty is not to obtain a surface ready to paint but to eliminate all high and low spots, to produce a fair surface.
We do not try to go any further than 80 grit with the QuickFair. The final fairing will be done with sanding primer also called high build primer.
The high build primer will cover sanding scratches, small imperfections but will not fill large low spots. You have to decide when your surface is fair and ready for the primer.
The primer can be applied with a roller or sprayed on. Spraying is much faster and can be done with an inexpensive HVLP spray gun. We recommend our System Three primer. It is 100% compatible with epoxy resins. Our S3 primer is an epoxy resin.
During the building of this boat, we assumed that you used quality resins like the ones sold at BoatBuilderCentral.com. Most of our resins have a low amine content or no amine blush. We also recommend to work wet on wet and this eliminates most amine problems.
However, if you use other resin brands or wait several days between fiberglass work, you must take in account amine blush.
Amines are part of some epoxy hardeners. During the cure, part of those amines may not combine with the other components and migrate towards the surface.
As long as the epoxy cross linking reaction is happening, amines will continue to migrate to the surfaces. That reaction (cure) can take several days depending on the temperature.
This means that you can apply a coat of epoxy on a previous coat that is hard but only a few hours old without worrying about amine blush because the amine will continue to migrate through the new coat.
However, if the cure is complete, let's say 2 days or more, there maybe a thin film of amine residue on the surface. That amine blush will prevent further layers of resin or paint to bond properly to the substrate.
In that case, you should lightly sand the surface and wash the amine off with a wet rag or sponge.
How important is it? It depends a lot on the resin. Our System Three Silver Tip and our MarinEpoxy with slow hardeners will not blush and do not require any removal of the amine. Our other resins will have a small amount of blush and should be washed off if the last coat has cured since more than 24 hours. Some major brand name expensive epoxies produce a strong amine blush. In case of doubt, sand and wash.
Amine blush feel and look like a light whitish greasy film. It is very obvious with some types of resins.
Again, if you work wet on wet or on one of our resins that is cured since less than one day, do not worry, apply the next coat be it resin or primer without sanding.
Back to the paint.
You should apply several coats of primer before starting to sand. This will build up some thickness to cover the imperfection.
Allow the primer to cure a few hours, not more than a day. The primer will become very hard in a few days and we want to sand before it reaches that stage.
Before sanding, we recommend to use either cheap spray paint or dye maker ink to coat the primer. That ?marking? coat will show low spots and the sanding paper scratches.
The picture above clearly shows that there is still a ridge along the fiberglass tape and some low spots. Those areas can be filled with more primer.
As you progress, reduce the grit of the sanding paper.
An amateur builder will probably be satisfied with 150 grit at the primer stage.
Some details like a seat locker with a lid should be finished before applying the final coats of paint.