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How to Bottom Paint a Boat on a Trailer

Ian Fortey by Ian Fortey Updated on June 11, 2022. In Boats

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Painting a boat isn’t something every boat owner will have to do. But if you have a boat that needs a little sprucing up, just for looks or to keep it in good working order, the bottom of the boat is often the most challenging part to paint. Luckily it can be done while it’s right on your boat trailer.

There’s a good chance if you’re looking up a guide like this you’ve never done it before. It’s one of those things people don’t think of until the time comes. And you’re stuck wondering “wait, how do I do the bottom?” Well, don’t worry, it’s not as hard as it may seem at first. There are just a few things you need to keep in mind.

How to Do a Proper Bottom Paint Job

A painted and clean boat runs more smoothly in the water. Things like barnacles and algae can hamper its performance and it will get worse over time. They can clog up outboard motors and also just make your boat look bad. You spent all that money on a boat, you want it to look good, right? Of course. But eventually even cleaning will come up short and a new paint job is in order.

Pick the paint you’re using ahead of time. Boat paint is by no means normal paint, you can’t just grab some old house paint. You have a couple of choices available in terms of kinds. It depends on how you use your boat. If it’s in the water all year around, you should look into something with a higher copper content, a hard-modified epoxy boat bottom paint. But if you store it over the winter out of water, there are good quality copolymer ablative paints.

The key thing is to make sure it’s paint specifically for a boat’s bottom. These are designed with antifouling paint properties to preserve their effectiveness. Anti fouling paint is designed to inhibit the growth of algae and barnacles. They will also be designed to release as little chemicals into the water as possible

Clean the Boat

First thing’s first, you’re going to need to clean the whole outside of the boat. This job is best done as soon as possible once your boat is out of the water and on the trailer. If the slime and build up is still wet, it is easier to get off. That doesn’t mean it’s easy to get off, just easier. If the algae and scum dries and bakes on you are going to be spending some time scraping it clean. As it is, you’ll still probably need to do that.

Power wash your boat as quickly as you can and blast off as much of the goop and sludge as you can.

Strip That Old Paint

Nine times out of ten, you want a fresh surface for painting. That means stripping the old paint. No matter how well you clean it, this will be better. And it’s absolutely essential if you’re switching paint types. If you’re using vinyl paint and the old paint was non-vinyl. It absolutely needs to be stripped. Otherwise your new paint won’t adhere.

If you are using the same kind of paint and the bottom surface of your boat is still in good condition, then you may be okay. But if the surface of your boat has a lot of scrapes and flaked paint, we recommend fully stripping it. Trying to fix chips and gaps with a new layer of paint will more likely result in a poor paint job that doesn’t last long.

Use a good, old fashioned paint scraper and clean the whole surface. You’re probably just looking to do the bottom here and that means you need to know where to stop scraping. You can use painter’s tape and make a tapeline that’s easy to follow. Line it up with the waterline of your boat to ensure a good transition between old and new.

Sand Your Bottom

Once the paint is stripped as well as you can get it, it’s time to sand the bottom of your boat. Make sure you have a dust mask and goggles before doing this. A particle respirator would be the best course of action here, and a vacuum unit to suck up the sanding dust. Not only do you not want to get wood or fiberglass in your eyes and lungs, but boat bottom paint is not a substance you want to ingest either. This stuff can be pretty toxic, so take proper precautions ahead of time.

You don’t need to go crazy doing this, you’re just getting the entire surface ready for new paint. Scuff it up evenly with some 80-grit sandpaper. Reach as much of the underside as you can get with the boat trailer in the way. Doing this by hand is a good idea. You don’t want to use something like a belt sander which will be too abrasive.

Jack It Up

Now’s a good time to raise your boat higher if you haven’t already done so. Get it a few feet off the ground and above the trailer using a boat stand. Raise the stern first and then secure it before heading to the midsection and repeating the process. This is to ensure you can reach the spots that the trailer was covering before. Use two boat stands on either side to ensure stability.

Prepare to Paint

You have your tapeline already, now you can tape off the rest of the boat. Through-hulls, transducers and any other metal parts should be taped off here. If you have an inboard, make sure propeller shaft struts are taped off as well.

At this point you may or may not need to apply a barrier coat. How do you know? Well, chiefly it’s just whatever you choose. That said, check the paint you’re using. If it recommends a barrier coat, use a barrier coat. It can protect the hull and also help prime it for the paint. At the same time, a quality barrier coat prevents blistering and corrosion.

Follow the directions exactly. You may need a second coat and you’ll need to let it dry before painting the actual paint. Barrier coats are typically fast drying but the instructions will give you precise details.

Paint

Make sure your paint is well mixed before starting. This is very important with boat bottom paint because of the copper in the mix. If it has rested for too long, the paint will separate. A quick stir with a stick won’t do the job here. Ideally, get this on one of those paint mixers for several minutes to get it as well mixed as possible. If you do it by hand with a mixing paddle or other method, make sure the end result is a uniform color. Expect to spend a good 10 to 15 minutes doing it this way. You can always pour half the paint into a paint tray and then mix it there before pouting it back into the can and repeating the process several times.

You can use a brush but a paint roller is faster for the actual painting. Start where you have the waterline taped off and work your way down the keel. Make sure you’ve read the instructions for the paint you chose to know if you need one coat or two. It’s probably just one, but two is possible.

Boat bottom paint dries quicker than you might expect. For that reason, you’ll want to work at a brisk and steady pace to ensure you get it done properly. Don’t forget that you’re going to need to save a little bit of the remaining paint for the spots under the boat stands.

Now would be a good time to remove that painter’s tape. You need to let the paint itself dry but the paint will make the tape stick as though it were glue, so best to peel it off now. Let the paint dry for as long as the manufacturer recommends. It’s never a good idea to rush this.

Change Position and Finish Up

Once the paint is dry enough, you can do a reposition of the boat. That means adjusting the stands so you can get to the spots you were unable to paint before. Lower the bow and paint what needs to be painted, then do the same with the stern. Finish any spots you may have missed before and you should be good to go at this point.

Bottom Painting Boat Dos and Don’ts

Jacks: There are plenty of ways to raise a boat, including very complex machines. You shouldn’t need to get too technical here though, boat jacks work fine. You’re just painting the boat, you’re not doing repairs or replacing an engine or anything.  Motorized boat stands and all of those things are not truly necessary. But if you are using something else, just proceed with caution and always follow directions.

Paint: Make sure you’re choosing the right, specialized paint for your boat. We mentioned there are two kinds earlier and the situations where they work best. You don’t want to choose the wrong kind of paint. For instance, the hard epoxy paint is for a boat that will spend all year in the water. If you take a boat with hard paint out of the water for storage, it will oxidize. In a matter of days, the paint will no longer be good. So choose wisely!

There are other paints that can be used for the bottom of a boat. Some are made with lower copper contents or are designed to be less toxic. Do your homework to find what will work best for you and your boat.

Drop cloths: Wherever you choose to paint your boat, make sure the area around you is prepared. Drop cloths to catch paint spills can go a long way towards keeping your drive way or the lot you’re working in clean.

The Bottom Line

Not every boat owner likes or wants to paint their boat on a trailer. The added work of repositioning the boat can be seen as a hassle. But this is also a quick and fairly easy way to do the job on your own at home. It all depends on how much time, money and effort you want to invest in getting the job done. If it’s something you want to tackle on your own just to prove you can do it, we’ve set you up with the basics to ensure a good job. Just remember to always defer to instructions on the paint you purchased. As always, stay safe and have fun.

About Ian

My grandfather first took me fishing when I was too young to actually hold up a rod on my own. As an avid camper, hiker, and nature enthusiast I'm always looking for a new adventure.

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