How to clean a lawn mower deck.

No one likes a crusty deck!

Sam writes…

”  You know how grass cakes up under the lawn mower deck.  Should this be scraped off with a putty knife (which in the case of my Hustler removes the paint), or does the caked grass protect from rust? What is the best way to preserve mowing deck from rust? What to do….?”

Gunk in the trunk.

If you have a mower and you use it, then grass “build up” is almost guaranteed.

So, as Sam so succinctly puts it; “What to do?”.


Leaving it on there won’t form a protective coating for the metal. To the contrary, the grass clippings will begin undergoing a process known as Aerobic Decomposition.

That’s where organic materials begin to decompose in the presence of oxygen. Significant amounts of heat and oxidation are released during the decomposition. Kind of like grass clippings in a compost pile.

See, when organic material is arranged to provide some insulation (like under a nice, dark mower deck in a nice, warm shed), the temperature of the material will rise to over 170°F during decomposition.

So what does that mean to your deck?

Well, under the right circumstances, cO2 can have a corrosive effect on steel. Long story short, it can damage your deck. Moisture fosters rust not to mention all those, weird, moldy things growing under there!


“I got a brand spankin new, beautiful, shiny 21″ wide walk behind mower. A red one. How do I keep grass and stuff from building up under the deck?”

Get out your pens and write this down:

Before putting your mower away, clean the underside of the deck off. With a brush or a garden hose or something. Every time. Do this and you will never have a “build up” problem.



So it turns out you’re a lazy lima bean like the rest of us. You may have neglected to clean the deck off a couple of times (for a couple of months) and now its not looking so good. Well, there’s nothing else for it so it’s time to start cleaning.

For this job, I recommend the following:

Safety Glasses

Work Gloves

A filter mask (for all the lovely airborne stuff that might get knocked free)

A hammer or mallet (cuz we’re gonna whack stuff) and a chisel and/or scraper and/or sand paper (you know why)

A brush or broom (to remove debris)

A garden hose or power washer (to soften things up a bit)

A cold drink (This is gonna take a while)

Your therapist on speed dial (cuz you just might snap)


We need to gain access to the deck. Safely!

If its a walk behind mower, just tip it back and secure it. If its a riding mower, you may have to remove the deck to make the job easier.

If you’re working on a walk behind mower, disconnect your spark plug before touching the blade.

Once your deck is ready to go, let’s put on those glasses and gloves and get down to it.

You may want to start by hitting the debris with a little water first. Give it a good soaking and let’s see what comes off on its own. There’s no harm in letting it soak for a bit.

Now, there’s really no special trick to clearing “build up” from your deck. Just knuckle bustin’ and a lot of scraping.

Using the chisel and hammer, begin carefully removing the debris. Start at the outer edges and work your way inward.

Remember, we’re not swinging for the fences here! We’re just chipping off the gunk. You may lose a little paint here. It’s going to happen, so don’t sweat it right now.

When the bulk of the debris is gone, there may be a little bit of stubborn stuff hanging on. Now would be a good time to break out the sand paper. Take your time and lets get this thing cleaned off.

If you’ve used water and the underside is wet, it’s a good idea to lower the mower back to the ground then let the deck blades run for a moment to dry the area off.

So, for all intents and purposes, your deck is now clear. Now we have to make a decision.

We can leave it “as is”. Staying up on it will keep you from having to chisel it clear again, but exposed metal means eventual rust, which means trying to replace your long since discontinued deck shell, which means you end up with the worlds slowest go-cart.

Or we go the extra mile and we re-surface the deck.


If you’ve decided to re-paint, the good news is that we don’t have to worry so much about matching color codes for the paint.

You can, of course!

But it’s the underside of the deck. Does it really need a showroom finish? Probably not. What it needs is a good, primer and sealant and that’s it.

Disclaimer: Of course there is more to re-finishing metal and the inter-webs is full of all kinds of helpful information on how to do it “show room right”.

But for those of you who go out on the weekends, wiping the mower down with a cloth diaper while you whisper to it; an authorized dealer can help you find the right paint…and perhaps some help.


Re-painting is more than adequate. But since we’re talking about alternatives here (preventative options, so to speak) there’s a pretty compelling argument to be made for applying a good, non-stick, graphite or polymer coating instead.

There’s no reason to go nuts on the price either. You can get a
decent 1 gallon can for around $35 and it’s still cheaper than a new deck.

And we’re talking about the same amount of work as re-furbishing a metal deck chair. Clean, sand and spray. There are tons of DIY videos on how to do this, so I won’t go into the step by step

Or you can take it to any number of automotive body shops. I’m sure they’ll be happy to take care of it for much less than it would cost to get your car done.

Regardless of what you end up choosing, just remember it was
painted when it was new, and it got this way over time. So, logic dictates that regardless of what you end up putting on the underside of your mower deck, it’s going to get this way again unless….

Yep. Say it with me…

Before putting the mower away, clean the underside of the deck off.

Every time.


It’s just a matter of biting the bullet and cleaning it off when you’re done. Consistency is the key.

I know a fellow who has two simple wooden ramps next to his driveway. Made them out of an old railroad tie.

Before putting his Craftsman away, he drives the front wheels up on it and hoses off the deck.

Easy, peasy, try not to sneezy.

Of course, there are multiple products out there that claim to keep grass from sticking to the bottom of your deck. Heck, a little Pam non stick spray’ll do just fine if you don’t mind stray animals licking your lawn at all hours of the day.

I’m sure they do a great job until they don’t. But nothing out there will work forever. Eventually, you’re going to have to clean the deck off.

From a service standpoint, be as lazy as you want! It’s like Doritos used to say “Eat all you want, we’ll make more!”.

Go on! Dirty that sucker up till the blades won’t turn. We (and the many small engine repair centers around the world) have an hourly rate and we are happy to charge a fair price to take care of it for you.

But for the rest of you , not doing a “double half gainer” into a vault full of gold coins like Scrooge McDuck – a little prevention is worth a lot of cure.

I can’t make it any “clearer” than that.



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Bill Brown

Bill is the head of content creation for the LawnMowerPros Blog and DIY section. He’s been in the Outdoor Power Equipment Industry for years and he’s still learning new things everyday. You can often find him creating featured articles, DIY guides, videos, graphics and much more.