Sanding and Painting An Old Sailboat

Its only when you’re done that you get good at something.

Huub and Daniela (2023).

Introduction

This post covers both the material and the techiniques for sanding and painting Jean-du-Sud’s hull. It relies on a few key articles and videos. It provides a summary of the techniques reported, the techniques used and some key lessons learned. As Jean-du-Sud had its hull already painted, restoring the gelcoat was not really considered, and so this post does not cover gelcoat restoration techniques.

I begin with some basic vocabulary, then summarize the steps and techniques used on Jean-du-Sud and then give details about each of the steps and techniques. A brief set of pictures showing the results follow. Those who are already familiar with paintjobs and sanding techniques will probably only be interested in the summary.

Painting Typology

I found the vocabulary related to painting and hull restoration to be confusting. What is the difference betwen gelcoat restoration and painting? What is the difference between topside paint and topcoat paint? Some vocabulary helps clarifying what painting a boat means.

Substrate: the raw material that is being painted. On a fiberglass boat, its fiberglass. On a wooden boat, its wood.

Gelcoat is the outermost layer of fiberglass petrified in polyester resin. Although gelcoat can be pigmented to give it color, it is not paint. It is colored fiberglass. Restoring the gelcoat is not painting.

Topside paint is paint designed for the part of the hull above the waterline. Topside paint is designed to last and protect from ultraviolet rays.

Bottomside paint is designed to be applied below the waterline. It usually has antifouling properties, and it is designed to decay over time. Bottomside paint will often be called « antifouling paint ».

Topcoat paint is a topside paint designed to be the outermost layers of the paint job. It is a « finishing paint ».

Undercoat paint is a topside paint meant to be between the gelcoat and the topcoat.

Primer is a thick paint designed to seal the hull and prepare for paint. It is usually epoxy based, for a better adhesion to polyester based gelcoat.

Summary of the Paintjob on Jean-du-Sud

Key information

• Temperature: 25°C-30°C during the day, above 20°C during the night;
• Time: roughly twelve days, with some spare days. 4 days to sand the hull, and roughly 6 days to paint the hull;
• Where: outside, with moderate winds;
• Technique: rolling (no tipping);
• Paint:
  • Two layers (one gallon) of Interprotect 2000E two parts epoxy based paint for the primer;
  • Four layers (two gallons) of Citadel color 300 (« white birch », semi-glossy) two parts polyurethane paint for the topcoats.
• Tools: mohair style rollers and later foam rollers; 36, 80, 120 and 240 grit sandpaper; a 4″ x 4″ electric sander, a dremel tool, one robust guy painting and sanding; one watchkeeper looking for drips or sagging.
• Costs: roughly 1700 CAD (1275 USD), the most important cost is the paint itself.
• Overall result: 7/10. It looks nice from 2 feet away and up. Closer, hull imperfections start to show.
• If I had to do it again: I would pay much more attention to the quality of the rollers.

Overarching Steps

The overall steps for sanding and painting are best summarized by Robinson (2024). The article discusses both painting and varnishing techniques, so only the painting part applies.

1. Clean the hull with a cleaner;
2. Sand the hull up to the gelcoat (coarser grit);
3. Fair the dips and cracks with putty;
4. Sand the hull (finer grit);
5. Apply epoxy based primer paint (one coat per day); I sanded only the final coat.
6. Apply polyurethane based topside paint (one coat per day); I sanded between the second and third coat (see the remarks below on rollers).

Painting or Gelcoat Restoration?

Sanding 101

Sanding is the action of flattening a surface by rubbing abrasive sandpaper. Abrasion removes some surface material. When sanding with long strokes, the surface is flattened. Prior to a paint job, sanding provides a better grip for the paint and improve the looks by flattening the hull. Here are four key ideas that helps sanding.

Sandpaper Grit: From Coarse to Fine

Grit size refers to the size of the sand on the sandpaper. The bigger is the grit size, the smaller is the sand size and thus, the less material is removed from the surface. A 50 or 80 grit is typical to begin scrapping a surface.

The implication of moving from coarser to finer grit is that the surface will need more than one sanding pass. The first sanding pass, with coarser grit, removes old elements (paint, dirt, etc.). The second and third passes removes the scratches, cracks or even sanding scratches from the previous passes.

On Jean-du-Sud, I had to remove all the topside paint to get to the original gelcoat. I removed the former paint and several impurities lodged in small cracks with 36 grit sandpaper. I then moved on to 80 grit, and then 120 grit prior to applying an epoxy primer paint. The first sanding pass looked awful. It removed all the paint, but the hull had a hairy, steel wool like texture. Moving to 80 grit sandpaper got the surface smooth, but it still had scratches made from sandpaper. The 120 grit removed those scratches, and the surface became as smooth as baby skin.

Coarser Grit: Looks is A Good Visual Cue

When to stop sanding? When wondering about moving to a finer grit, visual indicators are helpful. If you are removing paint from an old job, changes of colors, old cracks appearing or former repairs showing up are good indicators. For gelcoat, a loss of dullness and a restoration of brightness is a good indicator that you have removed enough. In both cases, this is when you should be switching from a coarser grit to a finer grit. Sanding is then no longer about removing material than about flattening the surface.

Fairing Happens Before Flattening

If there are any dips or cracks that needs to be filled with fairing compound, it is best to do it prior to the finer grit sanding. You will then be able to sand those repairs through the fine grit passes.

A good video on how to fair the surface is provided by Boats.com (2011, video below).

Finer Grits: Feel is A Better Indicator Than Looks

At finer grits, visual indicators will no longer help in detecting where sanding is needed (especially if you have poor sight). It is rather more helpful to just feel the surface with fingers. Any roughness, or any dimp will be automatically detected by the fingers. A good indicator as to when to stop is then when passing fingers over all the surface reveals no rough places.

At this stage, leaving patches of old paint (if any) can be fine, as long as it does not show any flakes or signs of detachment from the hull. If so, the remaining paint contributes to the flattening. It is most likely that surrounding areas are higher, thus the sanding removes very little material from the still painted area.

The Interlux webpage corroborates this idea (on a very well designed webpage for DIY paintjobs):

Remove all the paint that’s in a bad condition. This might mean you’ll have to remove all the paint down to the bare substrate. It’s ok to save any layers that are in good condition.

Interlux.com

An Industrial Sander May Be Too Strong … or Not

Which tools to use is really of personal tastes and strength. I began the sanding on Jean-du-Sud with a 4″ x 4″ mechanical sander with 80 grit sandpaper (picture left) and first thought it was too slow.

I then switched to an industrial 3″ x 21″ belt sander (picture on the right) with an 80 grit for maybe two hours and found it was too strong for the task. First, it is harder to control, meaning that you may produce unwanted scratches by inadvertence. Second, you can easily grind sand through the gelcoat without noticing. Third, it is harder to handle round parts of the hull with a 21″ belt. I have seen other boaters use a belt sander and they seem to have liked it (e.g. Huub and Daniela, 2023), but I reverted to the 4″ x 4″ sander with 36 grit and found what worked for me.

Painting 101

To Tip or not to Tip?

In the variety of write-ups and recommendtions availlable online, there is some consensual advice on painting techniques.

First, weather plays a role. One should avoid rainy days for the application of paint, or avoid too hot temperatures, as the paint will dry too fast (both on the boat and in the pan).

Second, all techniques emphasize the importance of adding thinner in the pan over time to compensate for evaporation. Hazzlegrove (1998), mentions one tablespoon every five minutes. (I used one ounce every 10 minutes.)

Third, all write-ups emphasize the importance of not applying too much paint at once. The key to a good finish is to avoid drips, brush strokes or excessive application in one coat. Fourth, the learning curve is quick. After one or two applications on a small boat segment (perhaps the transom), you can quickly figure out what works.

There is however contradictory information on what technique is best to use. Some references recommend tipping and rolling (e.g. Tips from a ShipWright, 2017; JamestownTV, 2011; SV Tritea, 2020) while others recommend rolling alone (e.g. Hazzlegrove, 1998; Florida Sportsmen, 2023).

Rolling and tipping is about first rolling paint on the hull and then smoothing the paint with a brushpaint to avoid drips or excess paint. The advantage is the ability to remove excess paint, or bubbles, with the brushpaint. Rolling paint is done by rolling only, with perhaps a thinner paint and a tighter control of the paintflow. The advantage is that it avoids brush strokes on the hull. There is compelling evidence, as tested by BoatWorksToday (2020, center video below), that there are little differences between both methods.

On Jean-du-Sud, I did the first sixth of the hull (one third of one side) trying the roll and tip method, but found that the rolling method worked better. Brush strokes were too visible. I sanded back the first sixth of the boat and then worked on rolling alone. In applying the last layer of topside paint, I ended up « rolling and rolling » some parts, meaning that I had one roller with paint, painting the surface, but I had another roller withouth paint, absorbing the excess when needed.

In the face of contradictory advice, I found it was best to focus on outcomes with the tools at hand and to figure out what worked best. All write-ups emphasize the control of paint flow and the avoidance of sagging. This is what should be looked for as an indicator of success. It is indeed easy to observe, so it may better to figure out what works better in your setup (temperature, experience and equipment).

Foam Roller or Mohair Roller?

There are also contradictory recommendations as to what roller to use. Hazlegrove (1998) recommends using « foam like » rollers (specifically, West Systems 800) while BoatworksToday (2020) recommends « mohair like » rollers. Both claim the superiority of the product to control the paint flow.

I began with mohair style rollers and it worked fine for the primer and the first topside layer. However, the temperature increased during the application of the second layer and the paint kept the spongy texture of the roller. This made me sand the layer once it cured and change to foam rollers. This was a game changer, as the application became much easier. If I had to do it again, I would use only foam rollers.

Foam rollers have a tendency to shatter if the paint is applied sideways. They furthermore become soaky after 6-10 applications, then leaving marks and bubbles on the hull. I changed the roller everytime any of those two events occured. At roughly 20 CAD (15 USD) for a pack of ten rollers, it is not a costly decision.

The size of the rollers matters. I found that a large (roughly 10″ wide) roller are not suited for painting round surfaces such as a hull. A smaller (roughly 4″ wide) roller yields better results.

Applying the Paint

The most useful resources for describing how to apply the paint is Hazelgrove (1998) and the Interprotect application manual (International Paint LLC, 2016). The video from Tips from A ShipWright may also be useful if you decide to go with a « roll and tip » method. Hazelgroove gives a pretty good definition of the rolling technique.

I would typically begin by mixing 12 ounces of paint for a single layer and then add roughly 1 ounce of thinner (and then more every ten minutes, varying with temperature). As rollers have a better performance at the beginning of their life, I would begin painting first at the beam, where imperfections can be the most visible.

Proper planning and organizing is key prior to mixing the paint. Once the paint is in the pan, painting becomes a non-stop operation, as the thinner evaporates and the paint starts to cure. The hotter the tempareture, the smaller is the time window. (Epoxy based paints require 20 minutes before they can be applied.)

Applying paint is first about spreading the paint load accross the hull. When the roller is somewhat empty, it is then about finishing the application to obtain a uniform look. It is really useful to look at the paint surface from various directions and angles, as the varying angles with the sun will help detect areas that are not painted, or that are sagging.

It is best to look back at the paint you applied five minutes earlier (or less, depending on the temperature). Sagging or drips will become apparent then. If the roller sticks to the previously applied paint up to a point where you have to apply more force, it is more likely that the foam roller will break if you attempt to correct the problem. It is then best to wait and sand the paint with light grit (120, 240) before the next layer.

What to Do With Imperfections?

You basically have two strategies. Either add another layer of paint or sand the problematic area. Adding paint will not make surface imperfections (unevenness) go away. It is best to sand. Difference in coloration is a sign that the paint thickness is not uniform on the hull. Adding a layer of paint may then be a good strategy.

On Jean-du-Sud, the second layer of paint had to be sanded because it looked too « bubbly ». That was a problem generated by the roller type I first used, but also because the of the warmer temperatures. It cost me a day.

Results

See the pictures below.

Have you enjoyed this text? Similar boatwork texts can be found in this section.

References

Boat.com (2011). How to Prepare A Boat for Painting, YouTube video retrieved online in June 2024 at this address.

BoatWorksToday (2016). How To Roll And Tip Alexseal Paint On Your Boat!, YouTube video retrieved online in June 2024 at this address.

_______________ (2023). Gelcoat Buffering and Care, YouTube video retrieved online in June 2024 at this address.

Florida Sportsmen (2023). Fiberglass Boat Painting Made Easy: Prep, Paint and Top Coat with a Roller, YouTube video retrieved online in June 2024 at this address.

Hazelgrove, R. (1998). Awlgrip Paint Job Using Rolling and NOT Tip Method, document retrieved online in June 2024 at this address.

Huub and Daniela (2023). Sanding and Painting the Hull, YouTube video retrieved in June 2024 at this address.

Interlux.com (n.d.). How to Fully Repaint Blistering Paint on GRP / FRP, document retrieved online in June 2024 at this address.

International Yacht LLC (2016). InterProtect For Prevention and Repair of Gelcoat Blistering, document retrieved online in June 2024 at this address.

JamestownTV (2011). YouTube video retrieved online in June 2024 at this address.

Robinson, P. (2024). Painting and Varnishing A Boat: How to Get the Perfect Finish, Practical Boat Owner, document retrieved online in June 2024 at this address.

Sailing Tritea (2020). Mistakes and Successes of Painting the Topsides of an Alberg 30 Sailboat, YouTube video retrieved online in June 2024 at this address.

Tips from a ShipWright (2017). How to Easily Roll and Paint, YouTube video retrieved online in June 2024 at this address.