I’ve been a procurement manager at a 190-person industrial facility for about six years now. My job covers a lot of categories, but the one that generates the most email threads (ugh) is rubber and plastic—O-rings, silicone sealant, plastic sheet, and the little jars of cleaner that show up as surprise line items.
When I first started signing purchase orders for these materials, I assumed the main job was comparing unit prices. Three budget overruns and one very expensive resealing job later, I learned a different lesson: for Dow Corning materials, the price per unit is usually the smallest number on the invoice. The real cost is in the rework, the wrong material choice, and the time your maintenance crew loses waiting for the right part.
This checklist is for people in similar roles—buyers, planners, or maintenance leads who order MRO supplies like these on a regular basis. It’s not a materials engineering class. It’s the practical list I run through before approving a PO.
The 5-Step Checklist
There are five things I check every time. Step 1 is the one most people skip, and it’s the one that saves the most money.
Step 1: Understand What the Part Is Actually Doing
Let’s start with the most common case. If you’re looking at a pump rebuild and the request says “replacement O-ring,” stop before you search for a part number. What is an O-ring? It’s a doughnut-shaped elastomeric seal that works by being compressed between two surfaces. It isn’t a washer. A washer distributes load. An O-ring blocks fluid or gas by filling a gland and staying in compression. That distinction matters because an O-ring has to fit its groove properly, and the material has to survive whatever it touches.
I learned this the hard way in 2023. We ordered a batch of “standard” O-rings for a chemical metering pump, and I didn’t ask what the pump was moving. The old rings were black, so we ordered black nitrile. They failed within three weeks. The pump was handling a solvent blend that attacked nitrile. The replacement cost wasn’t the O-rings—it was the two hours of line downtime plus the cleanup.
The same logic applies to sealants. When someone tells me to order “Dow Corning sealant,” I ask which product. Dow Corning 790 silicone building sealant is a specific product for weatherproofing exterior building joints. It’s not a general-purpose caulk, and it’s not the right choice for potting electronics or sealing a pump flange. “Silicone” is a chemical family, not a single material. If you don’t know what the material is sealing, where it’s installed, and what it’s exposed to, you’re guessing with someone else’s budget.
Step 2: Verify Dimensions with a Chart, Not a Worn Part
This step sounds obvious, but it’s where I see the most avoidable waste.
For O-rings, use an o-ring sizes chart. The chart gives you the cross-section and inside diameter for standard sizes like AS568A. Don’t measure the old O-ring with a caliper if it’s been in service for years. A compressed, heat-hardened, or cracked O-ring is not a reliable reference. The right way is to measure the gland—the groove the ring sits in—and then pick the size from the chart. It takes an extra ten minutes and prevents ordering a hundred rings that “look about right.”
The same principle applies to clear plastic sheet. If a machine guard needs a clear polycarbonate panel, don’t order by the old panel’s thickness alone. Check the unsupported span. A thinner sheet is cheaper per square foot, but it will flex or crack if the opening is large. We once replaced a scratched panel with a budget acrylic substitute because it was 30% cheaper than the polycarbonate we normally used. It cracked within two months. The replacement panel cost us $84. The labor to install it cost us $320. That was a false economy.
For sealant, the important dimension is the joint itself—width and depth. A tube of Dow Corning 790 only goes so far if the joint is deeper than the material can cure. Most overage problems aren’t because the crew spread sealant too thick, but because the joint wasn’t measured and the order was based on a guess.
Step 3: Select the Material Family Before You Compare Prices
I can’t tell you how many quote requests I get that simply say “silicone O-ring” or “clear plastic sheet.” Those are categories, not specifications.
For O-rings, the material has to match the environment:
- Nitrile (Buna-N) is a solid general-purpose option for oils and water, and it’s inexpensive.
- EPDM resists weathering and some chemicals, but it doesn’t hold up around petroleum oils.
- Silicone handles high and low temperatures well, but it tears more easily and has poor abrasion resistance.
- FKM (Viton) costs more, but it handles higher temperatures and broader chemical exposure.
If you pick silicone because it sounds premium and the application is actually mineral oil, you’re going to pay more and get a worse result. The vendor won’t tell you that, because they’re not the ones signing the rework ticket.
For construction sealant, check the product data sheet before requesting quotes. Dow Corning 790 silicone building sealant is designed for exterior joints that need long-term weather resistance. That’s a different product from an interior acoustic sealant or a sanitary silicone. We keep 790 on our standard list for facade repairs, but I still make our maintenance lead confirm the substrate and the joint movement range before I send the PO.
And don’t overlook the prep materials. Dow Corning silicone cleaner is on our standard list for a reason: it’s part of the installation, not an optional extra. I’ve seen crews try to prep a joint with plain solvent because the cleaner was “too expensive” for the job. The sealant failed, and the repair cost far more than a few extra cans of cleaner would have. The people who actually install sealant will tell you surface prep is the difference between a twenty-year seal and a two-year seal. Listen to them.
Step 4: Calculate Total Cost, Including the Labor and the Redo
This is the step where I do my actual job.
Let’s use a simple example. A $6 O-ring fails and causes a pump seal leak. The production line stops while the pump is disassembled. Two maintenance techs spend 90 minutes on the repair. Your labor burden is say $70 per hour per tech. The total cost isn’t $6—it’s $216 in labor plus the coolant or fluid that leaked.
When I compared quotes for a $4,200 annual contract on silicone materials last year, one vendor’s cartridge price was 9% lower than the incumbent’s. But the cheaper quote excluded freight and required a minimum order quantity that would have left us with nearly two extra cases of sealant. Once I added the freight and the shelf-life risk, the “cheaper” option was actually $340 more over the year.
Silicone products have a shelf life. O-rings harden. Sealant tubes cure inside the cartridge if they sit too long. If you buy a pallet of material to get a volume discount and only use half before it expires, you didn’t save money. You paid to store scrap.
For clear plastic sheet, the calculation is different. The material isn’t expensive, so people stop thinking. But every time a maintenance tech cuts a panel to the wrong size, you’ve paid for the material, the wasted time, and the second attempt. If the supplier can cut to size or if the sheet has a protective film, those small conveniences are not “marketing add-ons.” They reduce the labor cost on your side.
Step 5: Document the Decision for the Next Order
This is the least glamorous step and the one I’m most likely to forget. I’ll admit it—I’ve approved orders based on a note that said “same as last time,” and later found out “last time” was wrong.
In Q2 2024, our maintenance lead asked me to reorder Dow Corning 790 silicone building sealant for a facade repair. The system still had the product code from 2021, and the quantity looked reasonable. If I hadn’t checked the project log, I wouldn’t have noticed that the 2021 order was for a different joint depth. The new repair needed 40% more sealant. The line item was approved, but the order was short. That meant a second order, a second freight charge, and a crew waiting on site.
Now every rubber and plastic order gets logged with three things: the measured dimensions, the intended service condition, and the product family. It takes five minutes. It saves me from re-asking the same questions every time.
Notes and Common Mistakes
A few things I still see go wrong, even when the steps above are followed:
Mistake 1: Treating the cheaper alternative as the same material. For O-rings, a “silicone rubber” from one supplier may have different durometer and compression set than a silicone compound from another. The product name is not the full specification. Ask for the datasheet or the durometer if the application is remotely demanding.
Mistake 2: Ordering the sealant and forgetting the cleaner. If a job calls for Dow Corning 790, the surface prep is part of the scope. Dow Corning silicone cleaner is our default for removing oils, dust, and old release agents before re-sealing. If you buy the sealant but not the prep materials, you’re saving $40 and risking a $2,000 call-back.
Mistake 3: Using the old part as the source of truth. A compressed O-ring, a yellowed plastic panel, or a cracked sealant bead don’t tell you the correct answer. Measure the space, look up the chart, and order from the specification—not from the worn-out sample. I say this from experience, not theory. I once ordered 250 O-rings based on a caliper reading of a flattened ring. I had to reorder the entire batch.
This system isn’t glamorous. It won’t win a cost-cutting award in the first month. But over six years of tracking orders in our system, it’s the difference between buying materials and buying problems. The vendor with the transparent quote and the complete spec sheet is the one who actually costs less. That’s a lesson I’m not planning to learn twice.