Pontoon Boat Winterization Checklist — Every Step Before Storage

Every fall, my pontoon boat winterization checklist gets taped to the inside of my garage door before I touch a single thing on the boat. I learned that the hard way after skipping a step in my second year of ownership and cracking a water line fitting because I forgot to drain the live well completely. That repair ran me $340 at the marina in the spring, which is more than my entire DIY winterization supply cost. So now I do it right, in order, every single time. This guide covers everything I do to put my 24-foot tritoon away for the season — engine, plumbing, battery, interior, and all the stuff people forget until they’re staring at the damage in April.

Engine Winterization — The Most Critical Steps

This is where people make the most expensive mistakes. Seriously. Get this section wrong and you’re looking at a top-end engine rebuild or a seized lower unit come spring. I run a Mercury 150 four-stroke outboard, so my steps here are specific to outboards, but I’ll cover sterndrive (I/O) differences too.

Fuel Stabilizer — Do This First

Before you do anything else to the engine, add fuel stabilizer to the tank while it still has a decent amount of fuel in it. I use Star Tron Enzyme Fuel Treatment — a 1-ounce bottle treats up to 16 gallons, and a 4-ounce bottle runs about $9 at any big-box sporting goods store. Run the engine for 10 minutes after adding it so the treated fuel works its way through the entire fuel system. Do not just dump it in a full tank and call it done. The stabilized fuel needs to reach the carburetor or fuel injectors.

For outboards, I also drain the VST (vapor separator tank) if my engine has one. Mercury four-strokes do. Check your manual — some engines have a drain screw on the VST; others need a fuel line disconnected at the engine and the primer bulb squeezed until dry.

I/O engines have an additional step: you need to run stabilized fuel through the carburetor until the engine dies from fuel starvation. This purges raw fuel from the float bowl so it doesn’t varnish over winter. It sounds counterintuitive — killing the engine on purpose — but it’s the correct procedure for carbureted sterndrives.

Fogging Oil — Don’t Skip This

Burned by skipping this once, I now buy a full can of CRC Heavy Duty Corrosion Inhibitor (the marine-grade “fogging oil” version, part number 06039) every fall. It costs about $12 a can. With the engine warm from running the stabilizer, remove the flame arrestor or air intake cover and spray a steady stream of fogging oil into the intake while the engine is running at idle. The engine will smoke heavily. That’s normal. Then shut the engine off mid-spray so the cylinders are coated with oil while sitting. This prevents cylinder wall corrosion over the storage period.

Four-stroke outboards also need a full oil and filter change before storage — not in spring, in fall. Old oil contains combustion byproducts and acids that will corrode engine internals if they sit all winter. I use Quicksilver 4-Stroke Marine Engine Oil 10W-30, a 6-quart kit with the filter runs about $45. Takes 20 minutes.

Lower Unit Gear Lube

Drain and refill the lower unit gear lube every fall. Always. If you pull the drain plug and see milky or foamy fluid, you have a water intrusion problem — usually a failed seal — that needs to be addressed before spring. Fresh gear lube is cheap. A gear lube pump kit and a quart of Quicksilver High Performance Gear Lube runs about $22. Water in the lower unit that freezes will crack the housing. That repair starts at $600.

Water System and Plumbing

Probably should have opened with this section, honestly, because it’s the one most DIYers rush through or forget half of. There are more water-holding components on a pontoon than most people realize.

Live Wells, Bilge, and Wash-Down Systems

Start by running all your pumps to empty what you can mechanically. Then use a wet/dry shop vac to pull the remaining water out of every sump. The bilge on most pontoons sits low in the toon tubes — check both sides if you have a tritoon. A standard shop vac with a narrow crevice attachment gets most of it.

After vacuuming, blow compressed air through every water line fitting. I use a basic 3-gallon portable air compressor and blow through the inlet, the outlet, and any crossover lines. Don’t skip the wash-down system if you have one — those 1/2-inch lines trap water in every low spot and joint.

RV Antifreeze — The Right Kind

After blowing out the lines, I pour RV/marine antifreeze into any line I can’t guarantee is fully dry. This is not automotive antifreeze — do not use that. RV antifreeze is propylene glycol-based and non-toxic. Camco brand, the pink stuff, is about $4 for a gallon at Walmart. Pour it into the live well drain, the bilge sump, and any trap-style fitting. It doesn’t need to circulate — it just needs to sit in the low spots where residual water collects.

The Ballast System — Most Forgotten Step

If your pontoon has a ballast system for wake enhancement, this is the component people forget most often. The bladders and connecting hoses hold water. Inflate the bags fully with the pump, then drain completely, then blow compressed air through the fill lines. If your system uses weighted ballast bags rather than water bladders, pull them out and store them inside — freezing water inside a ballast bag will split the seams.

Battery Removal and Storage

A marine battery left on the boat in an unheated garage or uncovered slip over winter will be dead or severely degraded by spring. This is not speculation — I killed a two-year-old Optima Bluetop doing exactly that before I knew better.

Disconnecting and Cleaning

Disconnect the negative terminal first, then the positive. Use a wire brush terminal cleaner (about $6 at any auto parts store) to clean both terminals and the battery posts before storage. A light coat of CRC Battery Terminal Protector spray keeps corrosion from forming during storage. If your boat has a battery switch, turn it to the “off” position even with the battery removed — it protects the switch contacts.

Trickle Charger and Storage Temperature

Store batteries somewhere that won’t drop below 32°F. An interior basement shelf is ideal. I use a NOCO Genius 5 smart charger — about $40 — connected to each battery in float/maintenance mode all winter. This charger is smart enough that it won’t overcharge; it just keeps the battery at full charge. A properly maintained marine battery lasts 4 to 6 seasons. One that’s left to discharge over winter might give you two seasons before it won’t hold a full charge. The math is obvious.

Label your batteries with the installation date if they’re not already marked. It removes the guesswork when you’re wondering whether to replace or recharge in year three.

Interior and Furniture Protection

The pontoon deck and furniture represent a significant chunk of the boat’s value. Twenty minutes of prep here prevents hundreds of dollars in UV and moisture damage.

Vinyl and Upholstery

Clean the vinyl thoroughly before applying any protectant — applying protectant over a dirty surface just seals the dirt in. I use 303 Marine Aerospace Protectant after cleaning with Star Brite Ultimate Vinyl Clean. A 32-ounce bottle of 303 runs about $18 and covers every seat, the helm padding, and the bimini frame. It blocks UV and keeps vinyl from cracking over winter. Skip the Armor All — it uses petroleum distillates that degrade marine vinyl over time.

Moisture Absorbers and Rodent Prevention

Toss three or four DampRid FG50T Moisture Absorbers (about $6 each at hardware stores) under the seats and in the storage compartments. Mold is not a spring problem — it starts in October if the boat is closed up with residual moisture inside. Change them halfway through the winter if your storage area has high humidity.

For rodent prevention, I put Irish Spring bar soap in mesh bags in every storage compartment and under the helm. Sounds ridiculous. Works reliably. Mice don’t like the fragrance. I also block the bilge exhaust port and any other openings larger than half an inch with steel wool before putting the cover on.

Electronics — Pull Them If You Can

Chartplotters, VHF radios, and stereo head units take the cover off the helm mount and come inside for winter. The connectors get plugged with small rubber caps or wrapped in plastic. This isn’t strictly necessary for most modern electronics, but freeze-thaw cycles over four months are hard on display screens and connectors. My Garmin ECHOMAP Ultra lives in the house all winter. It costs $700 — the ten minutes to remove it is worth it.

Cover Options — Snap-On vs Shrink Wrap

A quality snap-on mooring cover for a 24-foot pontoon runs $350 to $600 and lasts 5 to 8 seasons with proper care. Shrink wrapping typically costs $12 to $18 per linear foot at a marina — for a 24-foot boat with a bimini, expect $350 to $500 per season. Shrink wrap wins on weather protection and fits perfectly regardless of your boat’s shape. Snap-on covers win on long-term cost if you store indoors or in a covered slip. I use a snap-on cover in a covered storage unit; if I stored outdoors in a harsh climate, I’d pay for shrink wrap every year.

DIY vs Dealer Winterization — Cost Comparison

Dealers charge $300 to $600 for a standard pontoon winterization package depending on your region, engine size, and what’s included. Some marinas in the Northeast charge up to $800 for a full-service winterization with shrink wrap included. Here’s what that actually gets you versus what DIY costs.

Typical DIY Supply Cost

Fuel stabilizer (Star Tron 4 oz) — $9
Fogging oil (CRC 06039) — $12
Oil and filter kit (Quicksilver 10W-30, 6 qt) — $45
Gear lube and pump kit — $22
RV antifreeze, 1 gallon — $4
303 Marine Protectant, 32 oz — $18
DampRid, 4 units — $24
Battery maintainer (NOCO Genius 5) — $40 (one-time purchase)
Snap-on cover (amortized over 6 seasons) — ~$75/year

Total recurring annual supply cost: approximately $134. Add the battery maintainer in year one: $174. That’s it.

What a Dealer Does That DIY Might Miss

A marina technician will sometimes catch things a DIYer misses — worn impeller housing, a stressed lower unit seal showing early signs of leakage, corroded electrical connections at the engine harness plug. They’re looking at dozens of boats a week and recognize failure patterns quickly. If your boat is older or you haven’t had a professional service in two or three seasons, paying for dealer winterization at least once as an inspection is a reasonable investment. They’ll document what they found. You’ll know what to watch for.

Time Estimate for DIY

My first year doing this solo took about four hours with a lot of manual checking. Now I complete the full winterization in two hours and fifteen minutes, working methodically through the checklist. The engine work takes the longest — about 45 minutes including the oil change and gear lube. If you’ve never done your own oil change on an outboard, budget extra time and watch your specific engine’s service video first. Mercury and Yamaha both have solid official tutorial videos on YouTube. Worth watching once before you crack the drain plug.

The $130 you save every year adds up fast. Over ten seasons, that’s $1,300 staying in your pocket — which almost covers a new outboard lower unit if you ever do need one. Do the checklist. Do it in order. Don’t skip the ballast system.