Starting a soft wash business or just upgrading your current setup usually leads to one big question: which comet soft wash pump is actually worth the investment? If you've spent any time on industry forums or Facebook groups, you know the name Comet comes up constantly. There's a good reason for that, but it isn't just about brand recognition. It's about having a pump that doesn't melt the first time it touches high-strength bleach.
Soft washing is a completely different beast than traditional pressure washing. You aren't relying on raw force to blast grime off a siding; you're relying on chemistry. But that chemistry—specifically sodium hypochlorite (SH)—is incredibly corrosive. A standard pressure washer pump would be dead in a week if you tried to run roof mix through it. That's where the specialized diaphragm pumps from Comet come into play.
Why Diaphragm Pumps Rule the Soft Wash World
If you're new to this, you might wonder why we don't just use a big electric 12-volt pump and call it a day. Those are fine for small DIY jobs, but for professional work, they can be a headache. They're loud, they draw a ton of battery power, and they tend to burn out right when you're in the middle of a big roof job.
A comet soft wash pump is typically a diaphragm pump, often the P40 or P36 models. These are "positive displacement" pumps. Instead of a spinning impeller, they use flexible diaphragms to move the fluid. The beautiful thing about this design is that the moving mechanical parts are mostly separated from the caustic chemicals you're spraying.
Because the liquid is contained within the heads and the diaphragms, the internal crankcase stays protected. Plus, these pumps can handle "dry running" much better than other types. If your tank runs dry for a minute while you're walking around the back of a house, a Comet pump isn't going to explode instantly.
The Legendary Comet P40
If there is a "gold standard" in this industry, it's probably the Comet P40. You'll see this pump mounted on more professional rigs than almost anything else. It hits that sweet spot of flow and pressure that makes it versatile for both roofs and siding.
It delivers about 11 gallons per minute (GPM) at roughly 300 PSI. Now, if you're used to pressure washers, 300 PSI sounds tiny. But in soft washing, 300 PSI is plenty. It's enough to shoot a stream of mix 30 or 40 feet into the air, reaching those high peaks without you needing to climb a ladder.
The high GPM is the real secret sauce. When you're rinsing or applying mix, you want volume. More volume means faster coverage, and faster coverage means you're off to the next job sooner. The P40 is usually paired with a small gas engine, like a Honda GX160 or GX200, via a gear reduction box. This setup is a tank. It's reliable, easy to pull-start, and won't leave you stranded like a dead deep-cycle battery might.
Handling the Heat (and the Bleach)
One of the biggest selling points of a comet soft wash pump is the material used in its construction. Specifically, the heads are often made of polypropylene or treated aluminum to resist corrosion.
When you're spraying a 3% or 4% SH mix on a roof, that stuff wants to eat metal for breakfast. Comet uses Desmopan or Viton diaphragms, which are specifically engineered to withstand those harsh oxidizers. If you've ever used a cheap pump and seen the seals turn to mush after two jobs, you'll appreciate the engineering that goes into a Comet.
That said, "chemical resistant" doesn't mean "indestructible." You still have to take care of it. One mistake I see guys make all the time is leaving the mix sitting in the pump overnight. Even the best pump will eventually fail if you let bleach sit and crystalize inside the diaphragms. A quick fresh-water flush at the end of the day is the difference between a pump that lasts five years and one that lasts five months.
Setting Up Your Plumbing Right
Getting a comet soft wash pump is only half the battle; you also have to plumb it correctly to get the best performance. These pumps are great at pushing water, but they aren't always the best at sucking it over long distances if there's an air leak.
You want to make sure your suction line is "overbuilt." If the pump calls for a 1-inch inlet, don't try to choke it down with a half-inch hose. Use heavy-duty, non-collapsible suction hose. If the pump can't get enough liquid, it'll cavitate. Cavitation sounds like you're pumping gravel, and it's the fastest way to ruin your diaphragms.
Also, don't forget the bypass line. Diaphragm pumps need a way to divert water when you let go of the trigger on your spray gun. This bypass line should go right back to your supply tank. This keeps the water moving and prevents the pump from over-pressurizing or overheating. It's a simple loop, but it's the lifeblood of a gas-powered soft wash system.
Maintenance Isn't Optional
I know, nobody likes doing maintenance. We'd all rather be out making money. But a comet soft wash pump is a precision piece of machinery. The most important thing you can do—besides flushing it—is checking the oil.
Most Comet diaphragm pumps have a clear sight glass or a plastic reservoir. The oil should look like oil. If it starts looking milky or like a vanilla milkshake, you've got a problem. Milky oil means a diaphragm has ruptured, and the water/bleach mix is leaking into the crankcase.
If you catch this early, it's a $50 fix for new diaphragms. If you keep running it with milky oil, you're going to destroy the bearings and the crankshaft, and then you're looking at buying a whole new pump. It literally takes five seconds to glance at the sight glass before you start your day. Do yourself a favor and make it a habit.
Is the Investment Worth It?
You'll definitely pay more for a comet soft wash pump and a gas engine than you would for a basic electric setup. But you have to look at it through the lens of downtime.
In the exterior cleaning business, your time is your most valuable asset. If you're at a customer's house and your $150 electric pump dies, you're stuck. You have to pack up, go home, order a part, and reschedule. That costs you hundreds, maybe thousands, in lost revenue.
A Comet setup is built for the daily grind. It's designed to run for hours in the summer heat without breaking a sweat. It gives you the "reach" to stay on the ground, which is safer for you and your employees. When you look at the cost-per-job over the life of the pump, the Comet usually ends up being the cheaper option because it just keeps on ticking.
Final Thoughts on Choosing Your Model
When you're ready to pull the trigger, think about the scale of your work. If you're doing mostly residential siding, the P36 might be plenty. It's a bit smaller and more compact. But if you're planning on doing large commercial roofs or apartment complexes, just go for the P40. The extra flow makes a massive difference when you're trying to rinse down a huge surface area.
At the end of the day, your pump is the heart of your rig. You can have the fanciest truck and the best-smelling soaps, but if you can't get the mix onto the surface, you're just a guy with a bucket. Investing in a comet soft wash pump is one of those moves that signals you're moving from a hobbyist to a professional. Take care of it, keep the oil clean, and it'll probably be the most reliable employee you ever have.