Many technicians believe that mixing filler on cardboard is an acceptable practice that saves time and money. The perception here is that you always have a clean sheet — but do you?
This discussion has followed me throughout my career. Early on in my career, I was taught to mix on cardboard without understanding the negative effects it can have on filler performance and repair quality. Beyond that, there is another level of impact: the lost time and money that mixing on cardboard can create for both the shop and the technician.
Let’s get straight to the question: Is mixing on cardboard wrong?
Yes. Yes, it is. Technicians should always use a disposable paper mixing board for mixing fillers and glaze materials.
Cardboard found in shops typically comes from shipping boxes for fenders, hoods, headlights, and other parts. Cutting off a flap seems convenient, but the cardboard itself can become the problem. That’s because shipping cardboard is treated with fire retardants, waxes, and mold release agents. These contaminants can transfer into the filler, affecting curing, adhesion, and overall performance. When I get calls about poor filler adhesion, soft edges that roll back, or material that won’t feather properly, the first thing I ask about is how and where the filler is being mixed.
“I don’t mix on cardboard — I mix on the filler I sanded smooth on the cardboard.”
If you’ve been in the industry long enough, you’ve heard something like this. Unfortunately, I have too. Years ago, as a young technician, we used to save the cardboard packaged with reconditioned wheels, spread filler on them, sand them smooth, and use them as a mixing surface. It even became a contest to see who could keep theirs the longest. Looking back, it’s embarrassing-but you don’t know what you don’t know, and as a mentee you assume your mentor knows best.
Body filler chemistry is unique. It bonds quickly to itself, whether cured or uncured. So, when you mix fresh filler on top of sanded filler, the styrene, resin, and BPO leach into the cured layer below. This changes your mix ratio and prevents a fully homogenous blend. As you scoop material off the surface and apply it to the repair, you run the risk of spreading improperly cured filler. If you’ve ever found soft spots during sanding or edges that roll back instead of feathering, this is often the cause.
I’ve always been a stickler for efficiency. I calculated ROI on tools, tracked my weekly productivity, and even wore a pedometer to compare steps walked to hours flagged. On my first day of college, my instructor Ron said something that has stayed with me: “You work for an employer, but your two stalls are your business. Run them like one.” As a technician, I wanted tools and materials I could rely on and products that made money, not problems.
That’s why it’s frustrating to see technicians sanding steel, aluminum, glass, or cardboard just to create a mixing surface. Shop owners should be frustrated too. Here’s why:
A disposable paper mixing board costs approximately $35 per 100 sheets, or $0.35 per sheet.
Now compare that to the cost of an 80-grit sanding disc, commonly used to sand down a “mixing surface.” Those discs typically range anywhere from $0.40 to $1.35 each. Before labor is even considered, sanding a cardboard board already costs more than simply pulling a sheet from a disposable paper mixing board.
Now factor in labor. If it takes 5 minutes to sand and clean that board, and an average repair needs three filler applications, that’s 15 minutes lost — possibly 30 minutes of productive labor displaced — plus three sanding discs. Multiply that by each technician, each day, each week, and the cost becomes substantial.
Disposable paper mixing boards can save time, save money, and help reduce the risk of quality issues that lead to rework and lost productivity. They also help keep the shop cleaner — especially important if you are not using dust extraction.
In my view, a disposable paper mixing board is one of the lowest cost, highest return investments a shop can make. If you’re not using them already, now is the time to start.
