Why Is Your Belt Sander Leaving Lines? 5 Causes and Fixes
Even experienced contractors get caught out by it: those frustrating belt lines showing up during or after wood floor sanding. You’ve got the machinery you trust and a full setup, you're running the right grit, but something’s off.
At Ultimate Floor Care, we speak to dozens of professionals each month about this exact issue. Whether it’s a quick phone call with a contractor on the job at that moment or a machine coming into our workshop for service, one thing’s clear: belt sander lines usually have a fixable cause. Here, from my experience, I want to go through the top 5 reasons for this. Now, these aren't all the reasons, but it would be a good start before you think about bringing your machine in for repair. Before you check these on the job site.
1. Dirty or Clogged Wheels: The Hidden Cause
This is one of the most overlooked causes of sanding issues. When wheels get coated in sticky lacquer and resin-rich dust (especially from pine or tropical timbers), they form an uneven layer that literally alters the wheel diameter. Imagine one side of your sander riding 10mm higher than the other. That tilt? It’ll transfer straight to your floor you're working on from the sanding path. Our servicing team, led by Simon, regularly sees machines that just need a basic clean, not a full repair. Prevention always costs less than correction.
What to do?
- Clean your wheels after every job or at least weekly.
- Use a filling knife to strip buildup.
- Avoid full wheel replacements by staying on top of this.
2. Alignment Issues from Transit or On-Site Handling
Most belt sanders are workhorses, but that doesn’t mean they’re immune to knocks. They spend a lot of time travelling from job to job and being pulled in and out of the van and the clients' homes. Driving over potholes, dragging machines across gravel paths, or throwing them in the back of a van unstrapped can knock key components out of alignment. Most commonly the wheels that are adjusted to cut parallel. What looked level in the unit now cuts unevenly in a client’s lounge. Simon’s seen it all and has designs we’re happy to share if you’re reworking your van setup.
How do you get around it?
- A properly built van rack to hold machines stable
- Always de-tension the drive belt before transit.
- Transport the machine decanted into it components.
- Invest in a transport cart to protect on bumpy ground.
3. Belt Tracking: Check Before It Bites
Tracking controls where the sanding belt sits, center on the drum and top roller. If this drifts to one side from vibration, knocks, or tension imbalance. It can cause the belt to fold or jam into the chassis. All it takes I a small lip on one side for the abrasive on your belt to start cutting grooves on the same side of the lip coming from the abrasive belt. Different machines have different ways of setting up ad adjusting. Contact your machine’s manufacturer or our team for help with your specific model. Belt tracking is a small fix that prevents major headaches later.
How to check if this is aligned?
- Run the machine off the floor with the door open.
- Watch for drift if it’s not centred, adjust or service it
- Contact your machine’s manufacturer or our team for help with your specific model.
4. Grit Choice and Drum Pressure: Don’t Skip the Steps
Drum pressure can be used to your advantage when used properly and there no process that works for every floor you need to gauge it and use it wisely. I've seen machines that have never been moved from full drum pressure having to use a wrench to crack the seized turn dial. Adjusting this can help avoid those lines in the first place. We get it, sometimes the temptation is to start aggressively. But jumping in with a 24 or 36 grit on a floor that doesn’t need it can cause more harm than good. Scope the work out and act accordingly, you'll be surprised on the time and abrasives you would save from starting higher. Every floor is different. That’s why we offer a drum pressure test during servicing to help you start with the right settings, not guesswork.
How to avoid this and use it?
- Use drum pressure wisely depending on the floor condition.
- Don’t Skip grits (e.g. 40 → 80) follow the correct process.
- Not adjusting for floor type (hardwood vs softwood)
5. Soft Timbers Like Pine Exaggerate Mistakes
Here’s the truth: sanding pine can be easy if tackled properly. It’s softer, more resinous, and far more likely to show belt lines if you treat if the same aggression as a hardwood like oak. Especially if your pressure, grit sequence or walking speed is off. Soft timbers need a gentler touch or they need the same consistent flow of process. The setup of your machine might be fine on an oak, but on a pine, due to the softness they can show up lines even when the machine is set up correctly. You can adjust your machine's wheels per job. Or switch to a rotary machine halfway through. And the right floor refinishing products can also help manage absorption and appearance. Most belt lines can be avoided with the right system, support, and care.
Some tips on pine mistakes
- Start finer (e.g. 40 or even 60 grit) if you can
- Set up your wheels and drum pressure depending on floor.
- Walk faster, don’t let the machine dwell.
- Spend more time on intermediate grits to blend properly.
At Ultimate Floor Care, we’re more than just a store. We’re your service team, your technical helpline, and your go-to supplier for professional floor sanding equipment, floor refinishing products, and dustless floor sanding solutions.
Need to sort a machine? Want advice on better floor coating solutions for soft timbers or commercial wood floor finishing? Contact us today.
