Wednesday, March 21

How to Bottom Paint - Tricks of the Trade

By Andrew Spaulding, Crowley’s Yacht Yard Asst. Production Manger

You might think that the differences in one bottom paint job to another wouldn’t add up to much and at one point in my life I would’ve agreed. But as it turns out there are a number of handy “tricks of the trade” that make a carefully applied bottom paint job better.

A Professionally Applied VC-17 Race Bottom
First off, taping and masking the areas where you don’t want the paint is the key to a professional looking job. It may seem like the taping and masking is a waste of time, but unless you are a professional painter (or artist) you won’t have enough control over the paint brush to keep the edge smooth and sharp. Of course, if I’m going to spend this much time discussing the need to tape, I’m going to have an opinion about which tape to use!

Just like many other tasks certain tapes are made for certain jobs. We use green tape (3M Scotch Automotive Refinish Masking Tape 233) for most taping jobs. It is conformable and hugs curves and contours pretty well. It doesn’t transfer adhesive to the hull as long as the tape is removed in a day or two. For longer term jobs (particularly outside) it is important to use weather-resistant tape. The advantage is that this type of tape won’t transfer their adhesive to the hull even if left on the boat for days in the direct sun. Regular household beige masking tape will leave adhesive residue very quickly particularly in the presence of moisture (dew) or high heat (sunlight). Our general purpose weather resistant tape is 3M Scotch-Blue Painters Tape 2090 which is commonly referred to as blue tape. According to 3M, blue tape is good for 14 days. In my experience, particularly if rain or heavy moisture is imminent, it is a best practice to remove the tape even if it has only been on for a few days. Once it gets wet, it is likely that the tape backing will separate from the adhesive upon removal leaving a sticky mess on the boat. Cleaning the adhesive off takes so long that I’ve seen many boat owners turn around and hire the boatyard to remove the adhesive in this situation. Trust me…it is less expensive to remove and re-tape the boat!

Now, we are onto the rest of the boat. Take my advice and tape or mask off everything. Unless you cut in with a paint brush for a living, this will make your life easier. Tape off anything that you don’t want to bottom paint by accident, for example, the depth sounder and the speed transducer. If you are using copper-based bottom paint, tape off any thing that isn’t made from copper. This will include sterndrives, saildrives, trim tabs, external strainers and any other external metals. The reason for this is that the copper in the bottom paint will react with the other metals if they are in contact. There are special paints made with special copper compounds or even better without copper in them at all to apply to these items.

Use an appropriate paint for painting underwater metals. Let us know if you need help finding one to use. Before painting under water metals, be sure to read and follow the manufacturer’s recommendations for surface preparation. When painting around metal items attached to the hull, the copper-based bottom paint should be isolated from the metal. You will see this in practice around sterndrives where there is a 1” stripe of hull showing before the bottom paint starts.

Don’t be afraid to think “out of the box” when it comes to masking off areas of your boat that you don’t want to bottom paint. For example, masking off the shaft is a pain…you have the opening in the hull to deal with and the strut and prop to cover up. Stealing the aluminum foil out of the kitchen on your way to the boat yard will solve this problem…just crimp the foil around the shaft and prop! Very easy to put on and take off.

The most popular bottom paints at Crowley’s are Interlux’s VC-17m Extra and Fiberglass BottomKote NT (click for data sheet). VC-17m Extra is high performance anti-fouling paint that is very popular with racing sailboats for its low-drag properties. It is becoming more popular with powerboats as they realize that a smooth, low-drag bottom means fuel savings. Fiberglass BottomKote NT is all-purpose bottom paint does a great job in this area. Both paints are quick drying allowing same-day launch or second coat. Before changing to one of these paints (or any bottom paint change) be sure to check Interlux’s compatibility chart as they will not go over all old bottom paints without incident.

As always, if you think about the cleanup process before the job starts you can put a little effort in early that will save tons of time at the end of the job. Be sure to cover the ground with a drop cloth to minimize the bottom paint’s impact on the environment. This step is critical if you are going to scrape or sand the bottom. We strongly recommend the use of vacuum sanders. Also, be sure to wear breathing, eye and skin protection while bottom painting…remember the stuff works because it is poisonous. Put two latex gloves on each hand, that way when you need a clean hand, peel off the outer glove and your hand is clean but still protected. Putting two gloves on will also offer protection should the outer glove get a hole in it. I am a particularly messy painter, so I actually put 4 gloves on my right hand so I can peel to clean several times during a job! By the way, this trick is also great to use when doing anything with 3M 5200.

The only surface preparation required for VC-17m Extra when it is applied over old VC-17m is that the surface must be clean and dry. Sand loose or flaking areas with 320 grit sand paper. Where needed (and after any sanding), clean the surface with Interlux Special Thinner 216. If Fiberglass BottomKote NT is your choice of paint, sand the entire surface with 80 grit sandpaper and wipe the surface clean with Special Thinner 216.

Choosing the correct applicator is very important when it becomes time to actually apply bottom paint. For VC-17m Extra Interlux recommends a solvent resistant roller cover with a short nap. In the Crowley’s Store we sell a red mohair roller cover that fits the bill nicely. It is important to remember to use a solvent-resistant roller cover because the solvents used in VC-17m Extra will quickly destroy a regular roller cover. Look for a phenolic or phenolic-coated roller for this purpose. When it comes to all-purpose bottom paint such as the Fiberglass BottomKote NT a standard 3/8” nap polyester roller will work fine. For brush work inside of thru-hulls and around fittings and other hard to reach places use any brush that fits the job. However, realize that the less expensive the brush, the more likely it is to leave bristles behind in the bottom paint. For the fussy bottom painter a higher quality brush is worth the money in less frustration.

VC-17m Extra dries very quickly. So much so, that it took a recent formula change to make it applicable with a roller. The practical issue that this causes is one where the paint will evaporate from the open can and paint tray very quickly leaving the tray and roller dry. So, be sure to only put a small amount of paint in the paint tray at a time and place the lid back on the can. If you still find the paint drying to quickly, thin the paint to slow the drying time with Interlux V172 thinner. It is important to use the correct thinner for particular bottom paint because different bottom paints use different solvents hence the need for different thinners. For example, Fiberglass BottomKote NT uses Interlux Special 216 or Brush-Ease 433 as a thinner. Be sure to follow the manufacturer’s recommendations when it comes to using thinners with bottom paint.

When it comes to painting thru-hulls, make sure the seacocks are open if you are going to try to paint up inside them. If the seacock (ball or tapered-cone valve) is closed, the layer of dried paint may prevent the valve from opening when you need it.
If you are going to paint on multiple days, the use of a paint tray liner will save your paint tray for another day without a ton of cleanup. At the end of the day, please do not dispose of liquid paint or solvents in the dumpsters or garbage cans due to fire and environmental hazards. Leave a project to do on the boat after the bottom paint is done, and before you know if the paint and solvents will be dry.

It takes time for the bottom paint to fully harden and it is prior to this that you want to pull the tape off. As soon as the last coat is tacky, pull the tape off carefully. It is possible that pulling the tape off will pull off some of the bottom paint if you allow the paint to harden. If this starts to happen, cut the paint along the tape edge very carefully with a razor blade. Continue to pull the tape carefully and cut the paint as you go to prevent the tape from pulling off the dried bottom paint.

So, that is it for now…you are ready to tackle the time-honored spring chore of bottom paint. If VC-17m Extra is your choice, this Saturday, March 24, 2012 at Crowley’s Yachtapalooza it will be on sale at a price so low we wince every time a quart is sold. Remember a great bottom paint job (like so many things) is possible only after great preparation work. As always, contact us with any questions. Feel free to reach me directly at

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